Intercropping Withcassava Proceedings of an International Workshop Held Attrivandrum, India, 27 Nov-I Dec1978 Ectors: Ecward Webec Barry \Estel Anc Ariyn Campdeh

$Intercropping Withcassava Proceedings of an International Workshop Held Attrivandrum, India, 27 Nov-I Dec1978 Ectors: Ecward Webec Barry \Estel Anc Ariyn Campdeh$

Intercropping WithCassava Proceedings of an international workshop held atTrivandrum, India, 27 Nov-i Dec1978 Ectors: Ecward Webec Barry \esteL anc ariyn CampDeH

ARCHIV 38421

IDRC-142e The International Development Research Centre is a public corporation created by the Parliament of Canada in 1970 to support research designed to adapt science and technology to the needs of developing countries. The Centre's activity is concentrated in five sectors: agriculture, food and nutrition sciences; health sciences; information sciences; social sciences; and communications. IDRC is financed solely by the Government of Canada; its policies, however, are set by an international Board of Governors. The Centre's headquarters are in Ottawa, Canada. Regional offices are located in Africa, Asia, Latin America, and the Middle East.

Weber, E. Nestel, B. Campbell, M. IDRC, Ottawa, CA Central Tuber Crops Research Institute, Trivandrum, IN IDRC-142e

Intercropping with cassava : proceedings of an international workshop held at Trivandrum, India, 27 Nov-i Dec 1978. Ottawa, Ont. IDRC, 1979. 144 p.: ill.

/IDRC publication!, !conference paper!, !cassava!, /intercropping!, !crop diversification!, !agricultural research!, /research centres!, !Latin America/, !Asia!, !Africa/ - !agricultural management!, !plant protection!, /soil fertility!, /economic aspects!, !CIAT!, !IRRI/, !agricultural statistics!, bibliography.

UDC: 631.584:635.23 ISBN: 0-88936-231-9

Microfiche edition available IDRC- 142e

Intercropping with Cassava

Proceedings of an international workshop held at Trivandrum, India, 27 Nov-i Dec 1978

Editors: Edward Weber,' Barry Nestel,2 and Marilyn Campbell3

Cosponsored by the Central Tuber Crops Research Institute (Indian Council for Agricultural Research) and the International Development Research Centre

1Senior Program Officer, Agriculture, Food and Nutrition Sciences Division. IDRC. 2Consultant to the AFNS Division of IDRC. 3Communications Division, IDRC. Contents

Foreword Edward Weber and Barry Nestel 5 Multiple cropping cassava and field beans: status of present work at the International Centre of Tropical Agriculture (CIAT) M. Thung and J.H. Cock 7 Intercropping with cassava in Central America Raüf A. Moreno and Robert D. Hart 17 Cassava intercropping in Brazil Marcio Carvaiho Marques Porto, Pedro Alves de Almeida, Pedro Luiz Pires de Mattos, and Raymundo Fonséca Souza 25 Intercropping systems with cassava in Kerala State, India C.R. Mohan Kumar and N. Hrishi 31 Cassava intercropping patterns and management practices in Indonesia Suryatna Effendi 35 Cassava intercropping patterns and management practices at Tamil Nadu Agricultural University, Coimbatore, India C.R. Muthukrishnan and S. Thamburaj 37 Cassava intercropping patterns and practices in Malaysia W.Y. Chew 43 Intercropping with cassava in Africa W. N. 0. Ezeilo 49 Cassava and cassava-based intercrop systems in Thailand Sophon Sinthuprama 57 Cassava intercropping research: agroclimatic and biological interactions H.G. Zandstra 67 Soil fertility implications of cropping patterns and practices for cassava Jerry L. McIntosh and Suryatna Effendi 77 Agroeconomic considerations in cassava intercropping research J.C. Flinn 87 Agronomic implications of cassavalegume intercropping systems Dietrich E. Leihner 103 Crop protection implications of cassava intercropping RaülA.Moreno 113 Discussion summary 129 Bibliography 133 Participants 141

2 Crop Protection Implications of Cassava Intercropping Raül A. Moreno Centro Agronórnico Tropical de Investigación y Enseñanza (CA TIE), Turrialba, Costa Rica

It has been cited repeatedly (Janzen 1973; Igboz- crop residues, shifting cultivation, rotation, and the wike 1971; and Hoidridge 1959) that tropical agro- cultivation of mixtures of varieties contribute to ecosystems with mixed stand populations are more achieve a certain degree of sanitary stability. Severe stable than monocultures, although this belief can parasite damage is in general terms rare in subsis- be refuted with several exceptions, notably sugartence production systems. As the farm becomes cane (Robinson 1976). Outbreaks of insect pests larger and the resource base more diversified, the and/or plant pathogens constitute one of the mostcassava-based production systems become more important factors contributing to the unbalance of market-oriented and cassava monoculture becomes a plant community in the tropics. prevalent. The management of these market-oriented The search for integrated pest management meth-production systems implies the use of pesticides al- ods has led to a series of entomological studies and though cassava is still considered a safe crop as com- theoretical discussions related to the influence of the pared to other tropical crops. composition of the plant community on the com- Small-scale agriculture has not received attention position of the insect populations. Some of these from researchers until recently; consequently data studies have been recently conducted under tropical on these mixed cropped production systems have conditions (Risch 1977). However, this information just begun to accumulate. is still not enough to provide a basis for designing Information about cassava diseases was generally new, more stable production systems for differentlacking (CIAT 1970) but has improved notably in tropical conditions. recent years (Lozano and Booth 1974). However, Information on the influence of mixed plantquantitatively epidemiological studies are few populations on the plant pathogen community com-(Lozano and Sequeira 1974) and most of these eval- position in the tropics is greatly lacking. Botanicaluated the reaction of different cultivars to certain epidemic development in most plant communitiescassava pathogens (CIAT 1975). of mixed genotypes has been studied mostly in tem- Information about botanical epidemics on cassava perate regions with horticultural crops (Berger 1973, intercropped with other crops was unavailable to the 1975) and more thoroughly discussed in cereals author and as a consequence, this paper is based (Krantz l968a, b, and 1974). Studies of botanical largely on Central American data on plant pathogens epidemic dynamics to interpret the hostpathogen- attacking cassava cultivated in pure and mixed environment system in its tridimensional magnitude stands and on the possible influence of cassava on are recent in plant pathology. Because tropical eco- the disease development of other crops that are inter- systems are generally more complex than temperate cropped with cassava. Most of the information has ecosystems and there are more plant diseases in the been obtained during the last 4 years by my former tropics than in temperate regions (Weilman 1969), graduate students, colleagues, and field assistants the study of botanical epidemics in the tropics isof CATIE. complex and needs a more integrated systems approach (Robinson 1976). Cassava, like other food crops, is intercropped by Influence of Intercropping on small farmers to minimize production risk involved Cassava Disease Development in "reduced scale" agriculture. These close to sub- sistence food crops production systems are culti- Larios and Moreno (1976, 1977) conducted one vated with the minimum of cash expenditure on pest of the most comprehensive studies on the influence and pathogen control. Intercropping, the burning of of intercropped species on the development of cas-

113 sava diseases. In this work the incidence and sever- Very susceptible cultivars display a necrotic reac- ity of several cassava diseases were studied in dif-tion. In Turrialba, both incidence (I) and severity ferent cropping systems (Fig. 1). Species, varieties(S) of this disease were positively correlated to de- of plant components, and planting distances in everyviations (frequency and amplitude) below the average cropping system are summarized in Table I. Inci- temperature (21.1 °C) and below average daily rel- dence (%) is the number of infected units andative humidity (88.79%). Significant and negative severity is the affected area (%) estimated accordingcorrelations between I and S and both average daily to previously calculated scales. The concepts of in-precipitation and 24-h periods with more than 5 mm cidence and severity have been previously discussedof rain, also were found. I and S decreased propor- by James and Shih (1973). Disease developmenttionally as the number of leaves per plant increased. was evaluated approximately every 20 days from theAs this disease attacks mainly the lower leaves, same 16 cassava plants randomly selected from 90when more foliage is produced either the relative m2 plots. humidity reaches levels inappropriate for pathogen development or the intercepted amount of light in- Cassava Powdery Mildew (Oidium manihotis P.creases to critical levels for both the pathogen and Henn) the persistence of lower leaves. Figure 2AE shows that peaks in both I and S curves coincide with the 0. manihotis attacks mainly the lower leaves of"dry period" at Turrialba (February and March susceptible cassava cultivars and produces a typicalmainly) when water tension reaches negative values yellowing on the attacked surface (Drumond 1946).(Amézquita 1974). Immediately after this period, either the growth of the host surpasses the development of the pathogen or the microenvironmental

CA SSA VA conditions become inappropriate for the pathogen. Maxima values of leaf area index in Turrialba are normally reached during AprilMay, about 185 CA SSAVA days after planting (Gallegos 1976). SWEET POTATO SWEET POTATO The incidence and severity of 0. manihotis attacks were higher when cassava was intercropped CASSAVA MAIZE with maize, than in cropping patterns not including I maize (Fig. 2, B and E).

CASSAVA Infection rates of severity reached 0.07 1 units per COMMON eEAN5 day in cassava intercropped with maize and 0.066 for cassava monoculture (Table 2). The other crop-

CA SSA VA ping patterns tested had lower infection rates. A sim- MAI2E SWEET POTATO ilar situation is represented by values of maximum COMMON aEANS severity. Sweet potato (!pomoea batatas L.) intercropped pOJ F1MIA1M,J 141A1 SO between cassava plants at the end of the cassava Months growing period did not significantly modify the de- Fig. 1. Cropping systems tested at Turrialba, Costa Rica, tovelopment of the epidemics (Fig. 2, D and E). The determine the influence of intercropping on cassava diseasesrate of decrease of disease of both I and S do not development (from Larios and Moreno /976). differ significantly.

Table 1. Species, varieties, planting distances, and density of plant components of five cropping systems including cassava, Turrialba, Costa Rica.

Planting distance (m) Planting density, Species Varieties Between rows On the row plants/ha Common beans (Phaseolus vulgaris) CATIE- I 0.5 0.2 100000 Maize (Zea mays) Tuxpeno 1.0 0.5 40000 Sweet potato (Ipomoea batatas) C- 15 0.5 0.4 50000 Cassava (Manihot esculenta) Valenca 1.0 0.5 20000

114 24- -60 0

20- -50 S

16 0 -40

0 a I2 I 30 I0 \ o U, 8- -20 S . 4 . -I 0 0 0 \ 4- 0 0 I I -10 "o 0 SI I .'It. J' 'M1 'M J J' 'M' M1 1J J1'M' j 'N''N' 1J B C 0 E Time( months Fig. 2. Severity and incidence values of cassava powdery mildew (Oidium manihotis) in different croppingsystems: A, cassava monoculture; B, cassava associated with maize; C, cassava associated with common bean; D, cassava associated with sweet potato; and E, cassava associated with maize, common bean, and Sweet potato at the end of thecassava growing period (adapted from Larios and Moreno 1976).

Common beans (Phaseolus vulgaris L.) inter-to the concept of Robinson (1973). Temperature planted with cassava delayed the onset and devel-could be higher due to reduced air movement and opment of both incidence and severity of 0. mani- light could be significantly lower in cassava inter- hotis infection (Fig. 2C). This is doubly importantcropped with maize than in cassava monoculture and because isariopsis griseola Sacc. infection was re-this could cause higher powdery mildew levels in duced on bean plants intercropped with cassavacassava intercropped with maize. (Table 8). A possible simbiotic relation between Cassava is relay-interplanted between maize these two plants components may be one of thestalks immediately after maize reaches physiologi- reasons why this crop association is widely practicedcal maturity to avoid high weed populations in the by small farmers. Assuming that intercropping cas-low humid tropics (Moreno and Hart 1978). This is sava and maize increases relative humidity in theone of the most widely cultivated cropping systems interplanted spaces, a low infection level is expectedin areas of high rainfall and luxuriant weed growth. since relative humidity is negatively correlated to IThe relevance of 0. manihotis in small cassava and S of powdery mildew. Perhaps infection oc- plants should be studied further in relation to this curred early in the growing season when maizepractice. Even though precipitation is abundant, plants were too small to modify the microclimatecassava plants growing with low light intensity be- and subsequent development of the epidemic oc- tween maize stalks are heavily attacked by 0. mani- curred by an esodemic type of process, accordinghotis during the early stages of cassava develop-

Table 2. Infection rates (r) of cassava mildew (Oidium manihotis) in different cropping systems, Turrialba, Costa Rica (from Larios and Moreno 1976).

Cropping system Infection rates Maximum severity, % Cassava 0.066 17.65 Cassava + sweet potatob 0.055 12.50 Cassava + maize 0.071 27.34 Cassava + beans 0.038 10.20 Cassava + maize + beans 0.07 1 19.27 aUflits per day (r of van der Plank (1963)). = cassava intercropped.

115 ment. The benefits derived from low weed infestation treatments involving maize showed the highest should be compared with the detrimental effect ofvalues for incidence and severity but this occurred high levels of powdery mildew. late in the growing season. Maize plants delayed the No data are available on the effect of intercrop- onset of botanical epidemics but, once the maize ping different cultivars of cassava and maize in dif- plants reached physiological maturity and were ferent, spatial arrangements on the development of doubled-over (about 135 days after planting), the the cassava powdery mildew. epidemic spread rapidly. This may be influenced by mechanical damage to cassava in the process of Cassava Scab (Sphaceloma sp.) bending the maize stalks, or by an increase in the number of propagules available for infection caused The symptomatology of this disease in Turrialba is characterized by scab lesions on the leaves,by field workers in contact simultaneously with infected and healthy tissue or both. The influence petioles, and young shoots. It is more similar to ear- of field workers on the spread of plant pathogens has lier descriptions of Bitancourt and Jenkins (1950) been previously noted by Berger (1973). It has been than to any other description. Elongation of inter- previously studied that wounds favoured Sphace- nodes as reported frequently elsewhere was seldom loma infection (Larios 1976). After the doubling of observed under field conditions. the maize stalks there is a large amount of suscep- Opposite to powdery mildew the scab pathogen attacks mainly during the rainy season (Fig. 3, A tible tissue available for infection. A similar case is discussed by Berger (1977) when disease severity through E). Maximum severity and incidence of the was less in sprayed fields. However, when spray disease usually occur a few weeks after the onset of applications ceased, disease sometimes reached the rainy season. The disease is present until gradual greater intensity in the previously sprayed fields than defoliation of the cassava plant, starting about 200 in the unsprayed areas because of the rapid disease days after planting, decreases both the amount of inoculum and susceptible tissue by reducing the progress following the loss of fungicide effective- ness. Table 3 shows the difference in infection rate number of leaves on the plant. of Sphaceloma sp. on cassava before and after dou- Accumulated precipitation and deviations of tem- bling the maize stalks. The higher value for "r"for perature (frequency and amplitude) above the mean the cropping system including cassava, maize, and (21.1 °C) were consistently and positively correlated common beans as compared to the cropping systems with incidence and severity of the disease in all crop- of cassava monoculture and cassava associated with ping systems studied. maize could also be a consequence of the field work- Intercropping with maize (Fig. 3, B and E) delays ers harvesting the bean plants and both damaging the onset of the epidemic significantly compared cassava foliage and possibly spreading inoculum. with monoculture, intercropping with sweet potato, or common beans (Fig. 3, A, C, and D). These last It has been observed in Turrialba that cassava scab three treatments had similar epidemic development increases more rapidly if tall maize cultivars, after curves for both incidence and severity. The twodoubling, remain as high as the cassava canopy.

0 0 0 0 00

'M' M1 'Jt A B C

Time( months Fig. 3. Severity and incidence values of cassava scab (Sphaceloma sp.) in different cropping systems. A, cassava monoculture; B, cassava associated with maize; C, cassava associated with common beans; D, cassava associated with sweet potato, and E, cassava associated with maize, common bean, and sweet potato at the end of the cassava growing period (adapted from Larios and Moreno 1976).

116 Table 3. Infection rates of cassava scab (Sphaceloma sp.) in three cropping systems before and after intercropped maize was doubled, Turrialba, Costa Rica (from Larios and Moreno 1976).

Infection rate (r)a

Cropping systems Before doubling After doubling Cassava 0.027 0.026 Cassava + maizeb 0.027 0.035 Cassava + maize + beans 0.04 1 0.048 aUnits per day (r of van der Plank (1963)). = cassava intercropped.

Wind movement of the hard dry maize foliage andfection was positively correlated with accumulated cassava leaves probably abrade the cassava and pro-precipitation, daily average precipitation, relative duce more wounds that favour Sphace/oma sp. humidity, and deviations (frequency and amplitude) entry. of temperature below average. Incidence was not Cassava scab is more prevalent on the foliage ex-correlated with precipitation or relative humidity. posed to the wind. In cassava monoculture the infec- Contrary to other cassava diseases, incidence and tion begins from the edge of the field facing the wind severity are not correlated. This disease, like several and spreads downwind in a decreasing gradient ofother rusts, apparently is not disseminated from in- severity. In cassava intercropped with maize thisfected to healthy host tissue under conditions of pre- gradient disappears after the first two rows of maizecipitation but either the size of the uredospore, the if cassava and maize rows are perpendicular to theproduction of new uredospores, or the production wind direction. Cassava scab spreads very slowlyof uredospores in existing lesions are favoured by when planted with plantain (Musa sp. ABB cv.available free water on the leaf surface. Pelipita) that is higher than cassava plants. If inter- Maximum intensity and severity of cassava rust cropping cassava with maize increases the amount are summarized for each cropping system studied in of powdery mildew but delays the onset of a cassava Table 4. Rust pustules were observed approximately scab epidemic, it seems logical to vary the maize 60 days after planting but the disease did not spread density in the field by increasing it in the area facing during the dry season. Cassava rust reached meas- the prevalent wind and decreasing it toward the cen- urable levels only 157 days after planting. Accord- tre of the field. ing to the maxima severity values, there was less rust Severity of cassava scab generally is moderate in in cassava cultivated in association with maize and Turrialba as compared with other regions in Central with common beans. As maize was already doubled America and elsewhere. It is not known if this ap- at this time, rust infection probably occurred early parent protection exerted by maize plants still holdsin the growing season when cassava was still asso- under very heavy infections. ciated with maize and the subsequent low incidence Cassava Rust Uromyces manihotis) and severity was due to small amounts of primary rust inocula in the plots planted with maize inter- This disease affected cassava under dry and hotcropped in cassava. The relevance of a reduced climatic conditions in Brazil (Normanha 1970) andamount of primary inoculum have been studied by in high altitude and cold areas in Colombia (LozanoBenson and Barker (1974), Summer and Littrell and Booth 1974). In Turrialba, severity of rust in-(1974), and Wiese and Ravenscroft (1975) among

Table 4. Maximum values of incidence and severity of cassava rust (Uromyces maniholis) in five different cropping systems, Turrialba, Costa Rica (adapted from Larios and Moreno 1977).

Maximum values of

Cropping systems Incidence (%) Severity (%) Cassava 67.7 2.85 Cassava -I- sweet potato 60.0 2.11 Cassava -4- maize 52.6 1.86 Cassava + beans 56.6 1.67 Cassava + maize + beans 47.2 1.17 = cassava intercropped

117 Table 5. Average values of severity of cassava rust (Uromyces manihotis) by canopy strata in different cropping systems, Tunialba, Costa Rica (adapted from Larios and Moreno 1977).

Average severity (%) by canopy strata

Cropping systems Low Medium High Cassava 7.43 1.09 0.04 Cassava + sweet potatoa 5.43 1.69 0.02 Cassava + maize 3.60" 1.40" 0.06" Cassava + beans 4.76" 083b 0.01" Cassava + maize + beans 4.20 0.55 0.04 Average 5.08 1.11 0.03 a + = cassava intercropped 5One disease assessment only. others. Maize acts as a barrier for bean rust sporeCercospora Leaf Spot (Cercospora henningsii) dissemination (Mora and Moreno 1978) and prob- and Cercospora caribaea) ably Ascophyta spores (Moreno 1975 a, b). It is also possible that cassava plants competing with maize Both C. henningsii and C. caribaea are minor in association during 130 days, due to its reducedcassava pathogens in Turrialba. No statistical dif- foliage might offer a smaller target leaf area for dis- ferences in either severity or incidence of the Cer- seminating rust spores. Considering the length ofcospora-caused diseases were found between the time of the total growing season, there were no sig- different cropping systems tested (Table 6). No ex- nificant differences in cassava rust infection rates planation is available for a more severe attack of between the different cropping patterns tested. C. caribaea on cassava plants intercropped with Similarly to powdery mildew, rust is more severebeans. A more detailed study should be conducted in the lower part of the cassava canopy. Rust wasin areas where Cercospora leaf spot is more impor- more severe in the lower section of the canopy than tant. in the upper, young, or middle leaf sections Cassava Dieback (Colletotrichum sp.) (Table 5). Humidity, temperature, age of leaves, light, and The intensity of damage (ID) of this disease was probably other factors, acting alone or in combina-studied in cropping patterns 1, 2, and 5 of Fig. 1 tion, modify the microenvironmental conditions in under low and high management levels. To evaluate different parts of the cassava canopy and signifi-ID the following formula was used: cantly affect the hostpathogen interaction. This in- (IAP) + 4(IMS) + 2(ISS) teraction should be studied for each host combina- ID tion in tropical areas where either powdery mildew or rust limits cassava production. In the low humid where: LAP = infected apical apices; IMS = infect- tropics Darlucca spp. parasites rust uredosporesed main shoots; SS = infected secondary shoots; earlier and with more intensity than in other tropical SP = susceptible points. areas; this is particularly true in cassava plants grow- The management levels differed by weeding fre- ing under plantain (Musa acuminata >< Musa hal- quency and fertilization (Table 7). hisiana) or cacao (Theohroina cacao). This is an im- Dieback always caused more damage under low portant biological control possibility worth exploring management, regardless of the cropping pattern through more detailed studies. (Fig. 4). According to Chevaugeon (1956), a potas- Table 6. Average number of lesions per leaf lobule per plant of Cercospora henningsii and C. caribaea in different cropping systems, Turrialba, Costa Rica (adapted from Larios 1976).

Avg no. of lesions/leaf lobule per plant

Cropping systems Cercospora henningsii Cercospora caribaea Cassava 0.28 0.39 Cassava + sweet potatoa 0.32 0.40 Cassava + maize 0.30 0.31 Cassava + beans 0.25 0.69 Cassava + maize + beans 0.24 0.35 a + = cassava intercropped.

118 Table 7. Differences in management of three cropping systems in which the damage caused byGlamerella cingulata was studied, Turrialba, Costa Rica (from Larios and Moreno 1977).

Management level

Low High

Amt of Amt of No. fertilizer, No. fertilizer, Cropping system weedings kg/ha weedings kg/ha

Cassava 300b_ 100' 2 300b.400e Cassava + sweet potato 300-100 2 300-500 Cassava + maize + sweet potato 300- 100 2 300-500 = cassava intercropped. b1530_8 (N-P2O-K2O) at planting. (N-P905-K20-S) applied 30 days after planting. sium deficiency may initiate dieback and Gloeos- Influence of Cassava on the Disease porium manihotis could subsequently grow in the Development of the Intercropped Plant affected tissue. The experimental area was low in potassium (Mojica 1975). If low soil potassium isCommon Bean Angular Leaf Spot (Isariopsis associated with dieback damage, higher infectiongriseola Sacc.) levels would be expected when cassava is intercropped with sweet potatoes and maize, which compete Common beans and cassava are commonly inter- for available potassium. There is an unproven pos- cropped by small farmers (Moreno and Hart 1978). sibility that a better soil nutrient balance occurs Snap beans produced as a cash crop in areas close when more diversified root systems are extracting to a market utilize some fungicides, notably dithio- nutrients simultaneously. carbamates, especially during the rainy season. Cas-

. Cossovci monoculture low management o a Cassova monoculture high management 2O Cassavo + maize + beans low management a A ACassava + maize + beanshighmanagement E -uCassavo +sweet potatoes low mogen*nt a D CCassavo + sweet potatoes high management

280 298 314 333 Days otterplanting Fig. 4. Intensity of damage of cassava diehack disease (Colletotriehumn sp.) under low and high level of inputs in different cropping systems, Turrialba, Costcm Rica, 1977.

119 Table 8. Severity of angular leaf spot of beans (Isariopsis griseola Sacc.) in different cropping systems, Turrialba, Costa Rica (from Moreno 1977).

Severity at different stages of development

Cropping systems Before anthesis Anthesis Green pod Beans l0.23' 14.37 19.56 Beans + maizea 10.31 17.77 21.33 Beans + cassava 10.81 13.61 18.88 Beans ± Sweet potato 10.26 13.13 18.89 Beans + maize ± sweet potato 10.46 16.11 21.03 Beans + maize + cassava 10.26 16.40 21.44 = association of crops; ± crop intercropped 30 days later. 5Data obtained through a modification of the McKinney index and transformed with (x + 0.5)2. sava benefits from this fungicide spray and fields plants from 1. griseola infection is not well under- intercropped with cassava and a cash crop frequently stood but perhaps could be explained by an um- appear free of diseases. However, in certain areas brella-like effect that diminishes the impact of rain no fungicides are used in bean production and the drops splashing propagules. Splashing water is an associated crops are also healthy. important factor in the dissemination of!. griseola The importance of some common bean diseases(Cardona and Walker 1956). Maize plants appar- when intercropped and in monoculture were studied ently do not protect bean plants from splashed pro- by Moreno (1977) and Mora and Moreno (1978). pagules of!. griseola, either because the infection The severity of angular leaf spot of common beans occurs early in the growing season when maize 20 days before anthesis, during anthesis, and at the plants are not fully developed or because the micro- green pod stage of development for different crop-climatic conditions prevailing in the interspace of ping systems are summarized in Table 8. cassava-beans is less favourable for the fungus than Bean plants growing between rows of either cas- the microclimatic conditions of the maize-beans in- sava or sweet potato were not as affected by I. gri- terspaces. It has been previously demonstrated that seola as were beans grown in monoculture and beans once synematas off. griseola are formed, sporula- grown in cropping patterns including maize. An-tion is more abundant during dark periods (Silvera gular leaf spot severity increased more before and 1967). Higher percentages of conidia germination during anthesis in treatments including maize among occurred under diffuse light conditions (Llanos their plant components. Between the anthesis and 1957). Maize plants could be providing a more ad- the green pod stage of development, angular leafequate microenvironment for 1. griseola germina- spot severity increased in the same proportion in tion, penetration into the host, and further dissem- every cropping system tested. Consequently the dif- ination than cassava plants do. ferences in severity registered between cropping Common Bean Rust (Uromyces phaseoli var. systems at the final assessment of angular leaf spottipica) is a result of a delay in the onset of the epidemic rather than in the further development of the disease. Bean rust severity was studied in the same crop- The specific role of cassava in the protection ofping systems as angular leaf spot but in different

Table 9. Severity of bean rust (Uromyces phaseoli) in different cropping systems, Turrialba, Costa Rica (Moreno 1977, unpublished reports).

Severity at different stages of development

Cropping systems Before anthesis Anthesis Green pod Beans 5 39b 15.11 17.16 Beans + maize 3.04 10.07 14.01 Beans + cassava 3.39 16.67 20.41 Beans ± sweet potato 3.14 16.66 18.79 Beans + maize ± sweet potato 0.71 11.04 15.67 Beans + maize + cassava 0.71 11.00 13.37 = association of crops; ± crop intercropped 30 days later. bData obtained through a modification of the McKinney index and transformed with (x + 0.5)02.

120 plots. Before anthesis, bean rust was present only COWPEA in beans grown in monoculture (Table 9). Traces of I COWPEA I rust infection were recorded in beans associated with cassava, maize, and sweet potato. Rust was not de- MAIZE MAIZE tected in the association of crops involving three COWPEA COWPEA components. During anthesis, beans grown with cassava and sweet potatoes were slightly more af- CASSAVA fected than the beans grown in monoculture. Lowest COWPEA LcwA severity values were registered in cropping patterns involving maize. During the green pod stage of de- PLANTAIN velopment, beans associated with cassava and with COWPEA ICOwPEA sweet potatoes had the highest severity. Cassava and sweet potatoes probably provided an adequate microenvironment for bean rust development. A re- JFLAMJ.JA, duced number of available infection sites for U. TIME (months) phaseoli due to higher severity of!. griseola on beanFig. 5. Cropping systems studied to determine the in- leaves could also account for less severity in beans fluence at cassava on the development of cowpea diseases. associated with maize. cropping systems (Fig. 5). Results for the first plant- Cowpea Mosaic Virus (CPMV) ing season show no difference between the number and Cowpea Chlorotic Mosaic Virus (CCMV) of cowpea plants infected by both viruses (CPMV and CCMV) in cowpea monoculture, in association CPMV and CCMV are major cowpea pathogens with maize, or in association with cassava. However, in Central America and elsewhere (Fulton et al. the number of cowpea plants infected growing in 1975). In Turrialba, both viruses are transmitted by intercrop with plantain was lower than the rest of the beetles of the family Chrysomelidae, notably Cer- treatments. The number of infected plants tended to otoma ruficornis, Diabrotica balteata, and D. add- stabilize approximately 60 days after planting in pha among others (Valverde et al. 1978). Dissem- cowpea intercropped between plantain (Table 10). ination of the pathogen in cowpea monoculture has There was a positive and significant correlation been frequently faster than among cowpea inter-between the activity of the vectors (D. balteata and cropped with maize (Gonzalez et al. 1975, 1976) butC. ruficornis) in the tested plots and percentage of data are inconclusive and variables such as adjacent virus infection in the cowpea plants. A negative cor- plot composition, unknown wild hosts of the virus,relation was found between activity of the beetles orientation of plots, weather conditions during the and both radiation and precipitation during the two growing season, percentage of seed transmission, growing seasons. Fig. 6-9 represent the activity of and several others are affecting the results in an un-the main vectors in plots with different cropping sys- determined manner. tems as determined by the number of beetles cap- An experiment was conducted to determine the tured daily in yellow-pan traps and the relations be- incidence (number of infected plants) in differenttween activity of the insects, precipitation, and

Table 10. Percentage of plants infected with cowpea mosaic virus (CPMV) andcowpea chiorotic mosaic virus (CCMV) in different cropping systems during the first plantingseason at Turrialba, Costa Rica (from Araujo and Moreno 1978).

Cropping systems

Days after Cowpea mono- Cowpea + planting culture Cowpea-I-maize Cowpea + cassava plantain 24 8.41 8.99 5.85 5.27 31 12.25 13.30 11.62 10.54 38 12.65 13.83 13.14 11.65 45 14.14 16.17 15.72 15.07 52 18.97 21.09 19.66 17.65 59 26.93 24.87 26.94 21.89 66 29.20 25.49 21.47 23.12 73 29.94 26.59 33.87 23.94 80 31.80 33.36 34.98 25.88 87 31.80 33.36 34.98 25.88

121 40

Cowpea monoculture Cowpeo + maize Cowpea+cossavo Cowpea + plantain

to-

o.l

Cowpeo monoculture 20- Cowpea+moize , C o Cowpea + cassava > 0. Cowpeo + plantain U, 00-to- a'.. . .; 0 C

200

tOO-

'S 0.4' Feb March Time (months) Fig. 6. Weekly activilyofDiabrotica balteata and Cerotoma ruficornis in different cropping systems and rainfall registered during the first planting season at Turrialba, Costa Rica, 1977 (from Araujo and Moreno 1978). radiation in two growing seasons. Risch (personal systems during two planting periods. The reduction communication) demonstrated that some beetles re- in virus infection did not result in a better cropping duce their activity proportionally under low light in- system as the cowpea yields were reduced about tensity. Cowpea was intercropped between cassava 60% when cultivated under plantain or cassava. stalks and plantain again during the second part of Higher yields have been obtained when cowpeas are the growing season (Fig. 5). There was a significant interplanted later in the growing season when cas- reduction in infected cowpea plants cultivated dur- sava has lost most of its foliage (Moreno and Hart ing the second planting season under cassava and 1978). More information is needed about the light under plantain (Table 11). intensity that allows an equilibrium between the Fig. 10 represents the radiation registered at the level that reduces vector activity but still allows for height of the cowpea plants in the different cropping an acceptable yield. Research on growing cowpea

122 30- - Cowpea monoculture Cowpea + maize Cowpea , cossava Cowpea + plantain

- S - 0 -u 1:.::-

- Cowpea monoculture - - --- Cowpea + maize 20- I.'.' a Cowpea+cassava ,. a. S Cowpeaplantain >U) fl / --\ 0o0 10-

200- I-- E

I, 100- "S C S '-S a S - S S S _--_ -- " I 'S/ 0-41 I June JulyI August Time(months) Fig. 7. Weekly activity of Diabrotica balteata and Cerotoma ruficornisin different cropping systems and rainfall registered during the second planting season at Turrialba, Costa Rica, 1977 (from Araujoand Moreno 1978).

Table 11. Percentage of plants infected with cowpea mosaic virus (CPMV) andcowpea chiorotic mosaic virus (CCMV) in different cropping systems, Turrialba, Costa Rica (from Araujo andMoreno 1978). Cropping systems

Days after Cowpea mono- Cowpea + planting culture Cowpea + maize Cowpea+cassava plantain 24 13.34 9.46 2.38 0.92 31 23.65 27.87 2.94 1.77 38 68.57 28.41 4.44 3.62 45 100.0 100.0 10.12 10.96 52 100.0 100.0 10.12 12.28 59 100.0 100.0 15.56 15.08 66 100.0 100.0 19.01 18.04

123 Cowpeo monoculture Cowpea+maize 30- Cowpeo + cassava - -- Cowpea splantain

0.._

3500

- 3000

2500- 0z 2000- Cowpeo monoculttre 0 Cowpea 4 maize 'S 4 Cowpeo +cassava 'S 'S 00- Cowpea + plantain S. 'S S.- l000 I I I I I I I JAN FEB MARCH

TIME (months)

Fig. 8. Average weekly activity of Cerotoma ruficornis and Diabrotica balteata in djfferent cropping systemsand radiation registered at the canopy of cowpea plants during the first planting season at Turrialba, Costa Rica, 1977 (from Araujo and Moreno 1978). under different degrees of cassava defoliation is con- a, b). Incidence and severity in number of units per tinuing. day of these two plant pathogens in different cropping patterns are summarized in Table 13. No significant differences exist between treatments in in- Ascochyta and Cercospora Leaf Spot of Cowpeacidence of Ascochyta leaf spot of cowpea but Ascochyta phaseolorum and Cercospora cruenta plantain, cassava, and maize intercropped with cow- are important pathogens of cowpea in Central Amer-pea reduced significantly the severity of the disease. ica. Ascochyta leaf spot was less prevalent whenNo significant differences either for incidence or cowpea was intercropped with maize (Moreno 1975severity were found for C. cruenta.

124 35., 0 0 Cowpea monoculture SI Cowpea $ mcize 25- Cow pea + cassova - , -S.. - - Coea +plantain U) In / t . g15 o , .. U > .- - / /'.'I SI .C ! SI 0 5- ! \.\ / ..--,,.-- .-...... -II ..-/ 3500- Cowpea monoculture Cowpea+maize Cowpea cassava 3000- -. - Cowpea + plantain

5, 2500-

2000

C 0 500-

N. 1000-

500-

0I I I I I I I I June July August Time (months)

Fig. 9. Average weekly activity of Cerotoma ruficornis and Diabrotica halteata in different cropping systems and radiation registered at the canopy of cowpea plants during the second planting season at Turrialba, Costa Rica, 1977 (from .4rau,io and Moreno 1978).

125 4000-

3500-

3000-

E U -S C U 2000- C 0 0 500-

1000-

Cowpea monoculture -- Cowpeo+ maize Cowpea +cossova 500- - - '- Cowpeo i plantain

I I I I I I I I 0 I I I I I I I I I ' Jon Feb March June July 'Aug Time (months) Fig. 10. Radiation registered at the heightofthe canopyofcowpea plants in different cropping systems and two planting seasons at Turrialba, Costa Rica, 1977 (from Araujo 1977).

Table 12. Time to the onset and final stage of an epidemic of Erysiphe polygoni D.C. in different cropping systems, Turrialba, Costa Rica (adapted from Araujo 1977).

Days to the onset Days to reach 100% Cropping systems of the epidemic incidence Cowpea 46 72 Cowpea + maizea 57 72 Cowpea + cassava 58 85 Cowpea + plantain 38 76 = association of crops.

126 Table13. Infection rates in number of units infected (incidence) and percentage of area affected (severity) of Ascochyta phaseolorum and Cercospora cruenta in different cropping systems, Turrialba, Costa Rica (adapted from Araujo 1977).

A scochyta phaseolorum Cercospora cruenta

Cropping systems Incidence Severity Incidence Severity Cowpea 0.0603k 0.0499 0.0582 0.0776 CowpeaH-maizea 0.0469 0.0183 0.0497 0.0737 Cowpea + cassava 0.0435 0.0238 0.0625 0.0732 Cowpea + plantain 0.0567 0.0294 0.0523 0.0629 = association of crops. 5Units per day.

Cowpea Powdery Mildew (Erysiphe polygonidelay in the onset of the epidemic by crop manage- D.C.) ment could represent a significant increase in cowpea yield. A significant delay of the onset of the ep- Cowpea powdery mildew is a disease with symp-idemic was achieved by interplanting cowpea with toms that are more prevalent at the end of the grow-maize, cassava, and particularly plantain (Table 12). ing season when cowpea is approaching maturity.Intercropping with cassava delayed up to 85 days the As chemical control is usually not economical, atime needed to reach a 100% incidence.

127