Plant Pathology (1998) 47, 243–251

Production of conidia by Peronosclerospora sorghi on crops in Zimbabwe

C. H. Bocka*, M. J. Jegerb, L. K. Mughoghoc, E. Mtisid and K. F. Cardwelle aNatural Resources Institute, Central Avenue, Chatham Maritime, Chatham, Kent ME4 4TB, England, UK; bDepartment of Phytopathology, Agricultural University of Wageningen, P.O.B. 8025, 6700 EE, Wageningen, The Netherlands; cSouthern African Development Co-operation/International Crops Research Institute for the Semi-Arid Tropics/Sorghum and Millet Improvement Program, PO Box 776, Bulawayo; dPlant Protection Research Institute, Department of Research and Special Services, PO Box 8100, Causeway, Harare; Zimbabwe; and eInternational Institute of Tropical Agriculture, Oyo Road, PMB 5320, Ibadan, Nigeria

Factors affecting the production of conidia of Peronosclerospora sorghi, causing sorghum downy mildew (SDM), were investigated during 1993 and 1994 in Zimbabwe. In the field conidia were detected on nights when the minimum temperature was in the range 10–19ЊC. On 73% of nights when conidia were detected rain had fallen within the previous 72 h and on 64% of nights wind speed was < 2·0 m s¹1. The time period over which conidia were detected was 2–9 h. Using incubated leaf material, conidia were produced in the temperature range 10–26ЊC. Local lesions and systemically infected leaf material produced 2·4–5·7 × 103 conidia per cm2. Under controlled conditions conidia were released from conidiophores for 2·5 h after maturation and were shown to be well adapted to wind dispersal, having a settling velocity of 1·5 × 10¹4 ms¹1. Conditions that are suitable for conidia production occur in Zimbabwe and other semi-arid regions of southern Africa during the cropping season.

In Zimbabwe and other countries in southern Africa Introduction there are reports of localized epidemics of SDM on Sorghum downy mildew (SDM), caused by Peronoscler- sorghum (de Milliano, 1992). Because of the arid ospora sorghi, is a serious disease of (Zea mays) conditions during the noncropping season there may and sorghum crops (Sorghum bicolor, Frederiksen et al., be no host for much of year. In these areas oospores, 1969; Williams, 1984). Plants may be infected either which are commonly formed in systemically infected systemically or with local lesions. Systemic infection sorghum plants (Williams, 1984), may be important in may arise from soil-borne oospores (Tuleen et al., 1980), initiating epidemics. However, if conidia are produced or aerially dispersed conidia (Cohen & Sherman, 1977; on these plants they may play a major role in infecting Ramalingham & Rajasab, 1981). Systemically infected later planted crops, as the conditions supporting conidia plants are usually barren. Yield loss has been shown to production are also conducive to conidia germination have a linear relationship with incidence of systemic and infection of the host (Bonde et al., 1985). In SDM (Craig et al., 1989). Local lesion infection, caused southern Africa there is a paucity of information on by conidia, occurs as discreet lesions on the leaf surface, the aerobiology and epidemiology of P. sorghi. If conidia and has not been implicated directly with yield loss, are produced and dispersed in large numbers from although it may act as a source of conidial inoculum for sorghum plants infected at the beginning of the season, subsequent systemic infection of younger plants (Cohen these conidia may play a major role in subsequent & Sherman, 1977). Young plants remain susceptible to epidemic development of SDM in southern Africa. systemic infection for only a few weeks after germi- In India and Israel long-term sampling of the air in the nation (Williams, 1984), and so in areas where crops are vicinity of crops infected by P. sorghi has produced ratooned or planted over a period of weeks there is a risk information on the pattern, frequency and quantity of of infection by air-borne conidia from early oospore- or asexual spore production in these regions (Kenneth, conidia-infected plants (Cohen & Sherman, 1977; 1970; Shenoi & Ramalingham, 1979). Conidia were Ramalingham & Rajasab, 1981). released for 2–6 h from 24·00 to 06·00 hours when the relative humidity was > 80%, leaves had surface water, *Current address: SRRC/ARS/USDA, 1100 Robert E. Lee and the temperature was in the range 18–23ЊC. Conidia Boulevard, New Orleans, Louisiana 70124, USA. from early planted, oospore-infected plants were found to be responsible for the bulk of infections as the season *To whom correspondence should be addressed. progressed (Cohen & Sherman, 1977; Ramalingham & Accepted 1 December 1997. Rajasab, 1981).

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Although conidia have been detected in the air ratoon crop from the previous season was situated 30 m adjacent to infected sorghum crops (Rajasab et al., north-east and was showing symptoms of systemic 1979), there is no information on how effectively they infection when spore sampling commenced on 24 may be dispersed by the low wind speeds in crop November 1994. canopies at night. The aerodynamic diameter of a Disease assessments of systemic and local lesion particle dictates its settling velocity and is a major infection were made in the adjacent crop experiment factor determining suitability of the particle for aerial areas on five occasions during both years. Systemic dispersal (Chamberlain, 1975; McCartney et al., 1993). infection was estimated by counting the number of The settling velocity of P. sorghi is unknown. infected plants in four replicate transects of 10 plants The following experiments were undertaken to each in all plots of the experiment, and calculating the investigate characteristics of the aerobiology of conidia mean value. The incidence of local lesion infection was of P. sorghi during the sorghum cropping season in semi- estimated by assessing the proportion of leaves infected arid southern Africa. Some factors affecting production on 10 plants chosen at random in each of the plots, and of conidia are investigated. The settling velocity of taking means. Both experiments were planted with the conidia is also calculated to confirm the suitability of var. Marupantse. conidia of P. sorghi for dispersal at low wind speeds. Weather data Meteorological data were obtained from the SADC/ Materials and methods ICRISAT/SMIP automated weather station situated in an open area at a height of 2·0 m. It was located Periodicity of conidia production in the field approximately 100 m distant from the Burkard sampler Spore sampling in 1993 and 800 m in 1993/4. Data were recorded on a A 7-day volumetric Burkard spore sampler (Burkard 4-h basis. Variables recorded included maximum and Manufacturing Co., Rickmansworth, Hertfordshire, minimum temperatures, and wind speed and direction. England) was used to detect conidia. The sampler was Daily records of rainfall were obtained for this site from operated in sorghum fields during 1993 (28 January to 3 the Department of Meteorology, Harare. April 1993) and 1993/4 (24 November 1993 to 31 March 1994) in Zimbabwe at the Matopos SADC/ Factors that affect production of conidia ICRISAT/SMIP (Southern African Development Co- operation/International Crops Research Institute for Temperature the Semi-Arid Tropics/Sorghum and Millet Improve- The effect of temperature on sporulation of systemically ment Program) farm. The air was sampled at a height of -infected leaf material was investigated. Thirty-day-old 1 m. The Burkard was run from a 12-V car battery, systemically infected container-grown plants (var. DMS which was replaced weekly. The sampler operates by 652), pre-inoculated at emergence, were held under light drawing air through an orifice at a rate of 10 L min¹1, overnight to condition them for sporulation (400 W high causing particles to be impinged on a rotating drum pressure sodium lights 19·00–07·30 hours). Counting immediately posterior to the orifice. The drum was from the first leaf produced, leaves 6, 7 and 8 were used. prepared by attaching a strip of melinex with double- Uniform systemically infected leaf material was removed sided sticky tape. A coating of vaseline wax (150 mL) from the plants, cut into 7-cm sections and placed and paraffin wax (18 g) dissolved in hexane was painted immediately in distilled water. Leaf sections were mixed on the melinex strip using a paint brush. This acted as randomly, wiped dry with tissue paper and placed the surface to entrap the spores. The drum rotates at abaxial surface uppermost, in Petri dishes lined with 2mmh¹1, each 48-mm section accounting for a 24-h moistened tissue paper. Four leaf sections were placed in period. The drum was changed every 7 days or less. After each Petri dish, with three replicate dishes for each sampling, the melinex tape was cut into 1-day strips temperature. The leaf sections were incubated in the (48 mm) and mounted on slides with a gelvatol/ dark at 6, 10, 14, 18, 20, 22, 24, 26, 30, 34 and 38ЊC. A methylene blue slide mountant (35 g gelvatol, 100 mL further set of dishes was kept aside to test for maturity of distilled water, 50 mL glycerol, 0·16 g methylene blue). conidia. Every 30 min, from 5 to 8 h, leaf sections were The numbers of conidia on the strips were then counted taken from these dishes and the conidia were tested for by microscopic observation at × 100 magnification. The maturity (conidia were assumed to be mature when spore count was made on an hourly basis by taking a freely released from the conidiophores). The experiment 100-mm traverse every 2 mm, and the number of spores was fully randomized. At maturity of conidia, the petri detected per m3 of air was subsequently calculated. dishes were removed from the incubator and the leaf In 1993 the sampler was located immediately south- sections placed in a desiccator jar containing 40% west (predominant wind direction from the north-east) formaldehyde to kill the conidia and prevent germi- of an experiment planted on 28 January 1993. In the nation. The leaf-pieces from each Petri dish were placed 1993/4 season the sampler was approximately 600 m in a bijou bottle containing 5 mL distilled water, and from the previous site, immediately leeward of an agitated vigorously to release conidia from the leaf experiment planted on 16 January 1994. An infected surface. The concentration of conidia was estimated by

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microscopic observation using a haemacytometer. The experiment was replicated three times. The microscope width of each leaf section in the bottle was measured and slides were removed from the MUI and mounted, and the number of conidia produced per cm2 of leaf was the number of spores deposited on each stage was calculated. counted by taking 8 × 50 mm traverses across each slide at × 100 magnification. The da was ascertained from the proportion (P) of spores detected on the different slides. Time period of release of conidia The estimate of d was made by taking that value of d Systemically infected leaf material was used to investi- a a that minimized the chi-squared value of P, where gate the time period over which conidia were released (P – P )2/P was summed over all the from conidiophores under still conditions. The leaf observed expected expected stages collecting spores in each replicate (McCartney material was selected and prepared as described for the et al., 1993). The physical diameters of 100 conidia previous experiment. The leaf sections were pinned counted from three 50-mm transects across a slide were abaxial surface uppermost to the inside of a rectangular also measured at × 400, and the mean compared with the plastic container (15 × 10 cm) lined with moist tissue estimate of the d from the MUI. paper. Enough leaf sections were used to cover the base a of the container. A sheet of plastic covered with a layer of moist tissue paper was prepared. A glass microscope Data analysis slide was placed on this layer, and the container inverted on the plastic sheet over the slides. Five replicate Statistica (Statsoft Inc., Tulsa, Oklahoma 1994) was containers were randomly placed on two shelves of the used to fit regression models to describe the effect of incubator at 22ЊC. After 3 h a fresh plastic sheet, temperature on the production and time of deposition of prepared in a similar manner, was placed under each conidia of P. sorghi. Prior to regression analysis the data container. This was repeated every 30 min for 11 h, from the conidia deposition experiment was log causing minimal disturbance to the container. Slides transformed, because of heterogeneity of the sample were mounted as described previously. The number of variance. For other data presented standard errors of the spores deposited was counted by microscopic observa- means were calculated. tion by taking the mean of six 100-mm transects across each slide at × 100 magnification. Results Conidia production from systemically infected leaves and local lesions Periodicity of production of conidia The spore production of field-grown systemically The results of the spore sampling in Zimbabwe in 1993 infected and local-lesion infected leaf material, and and 1993/4 are shown in Figs 1(a) and (b). Temperature container-grown systemically infected leaf material was data and rainfall for the Matopos site during this period investigated (leaf number 6–10). Two leaf sections (var. are also shown. Mean disease incidence in the adjacent DMS 652) were placed in each of eight replicate Petri fields is shown in Table 1. Conidia were detected dishes. The experiment was fully randomized. The intermittently in both seasons. In 1993 conidia were incubation period was 6·5 h at 22ЊC, as previous detected on 25 nights (sampling period 66 days), and in experiments had shown this to be the time at which 1994 on 33 nights (sampling period 130 days). In 1993 conidia achieved maturity (indicated by germination), conidia were detected 7 days before disease was and 22ЊC the optimal temperature for sporulation. observed in the crop. In 1993/4 many conidia were Methods to prepare and incubate the leaf material detected prior to planting the adjacent field, probably were as previously described. because of an infected ratoon crop of sorghum about 30 m distant from the sampling site. Few conidia were detected after mid-February 1994. Rain fell regularly The settling velocity of conidia during the 1993 sampling period, but in 1994 a drought The settling velocity was calculated from the aerody- started in mid-February, and disease incidence fell after namic diameter (da) of conidia, estimated using a May Julian date 55, as a result of plant death. Conidia were Ultimate Impactor (MUI, May, 1975), a cascade detected less frequently after this time. The number of impactor that separates particles of different settling conidia recorded during a particular day varied greatly velocity onto microscope slides. The slides are on within both seasons. During the different nights when different stages separated by different sized jets, which conidia were detected, the minimum temperature ranged sort the particles according to their da. The operation of from 10 to 19ЊC. the instrument was previously described by Bock et al. The production of conidia shows marked periodicity (1997). The apparatus was tested using Lycopodium (Fig. 2). Conidia were detected between 00·00 and spores to confirm that it produced results comparable to 08·00 hours only, the number being highest between previous reports (McCartney et al., 1993). Leaves 02·00 and 05·00 h, when mean wind speed was less producing mature conidia were shaken over a funnel than 2·5 m s¹1 and mean temperature was in the range attached to the MUI by a flexible hosing. The 15–17ЊC.

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Figure 1 Temperature (maximum, heavy solid line; minimum, light solid line; 4-h mean (20:00– 24:00 h) medium dashed line), rainfall (solid his- togram) and daily concentration of conidia (shaded histogram) of Peronosclerospora sorghi in the air adjacent to infected sorghum crops during the 1993 (a) and 1994 (b) growing sea- sons at Matopos, Zimbabwe. T, trace of rain; I, irrigation applied.

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Table 1 The incidence of sorghum downy a Mean disease incidence mildew in sorghum crops adjacent to spore Experiment Day of assessment Systemic infection Local lesions sampling locations at Matopos, Zimbabwe in year (Julian date) (% plants infected) (% leaves infected) 1993 and 1994 aDisease incidence averaged over all plots of 1993 adjacent area. b 50 3·4 (2·74) 12·3 (7·85) bStandard errors of the mean in parentheses. 63 4·6 (2·01) 39·9 (4·10) 81 5·9 (2·72) 57·0 (3·18) 96 7·3 (2·70) 70·7 (2·79) 120 7·2 (2·68) 84·7 (2·53) 1994 31 0 (0) 3·5 (2·64) 55 8·7 (1·30) 32·0 (5·70) 73 6·3 (4·66) 25·5 (7·69) 98 9·7 (3·15) 17·1 (10·13) 117 9·8 (4·95) 16·2 (9·40)

Conidia tended to be produced immediately after rain. rain, and on 8% of nights more than 6 days after the last The number of days with rain and the days when conidia rain. The greatest proportion of nights (81%) when were recorded are shown in Table 2. The fraction of the conidia were detected had a mean temperature between total nights on which conidia were detected and the 14 and 20ЊC (Fig. 3b). None were detected below 12 or number of days since the previous rain (both seasons above 22ЊC. For 64% of nights when conidia were found combined) is shown in Fig. 3(a). Conidia were detected the wind speed was less than 2 m s¹1 (Fig. 3c) and for 73% on 72% of nights between 1 and 3 days after the last of nights conidia dispersal lasted less than 5 h (Fig. 3d).

Figure 2 Mean diurnal periodicity of conidia production of Peronosclerospora sorghi adja- cent to a sorghum crop at Matopos, Zim- babwe during the sampling periods in the 1993 (shaded histogram) and 1994 (solid his- togram) growing season. Temperature (1993,W;1994,X) and wind speed (1993,A; 1994,B). Temperature and windspeed are based on 4-h means.

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Table 2 The number of days with rain and the number of days on which conidia of Peronosclerospora sorghi were detected during the spore sampling periods at Matopos, Zimbabwe in 1993 and 1994

Days with rain Days without rain

Days Days Total days Total number of days Number conidia Number conidia when conidia year with sampling of days detected of days detected detected

1993 66 24 19 42 6 25 1994 130 41 23 89 10 33

Factors that influence production of conidia by a third-order polynomial regression model. Max- imum deposition occurred 8 h after commencing incu- Effect of temperature on sporulation bation. No conidia were deposited with incubation The effect of temperature on sporulation of SDM from periods of less than 7 h, and deposition ceased after 10 h. Matopos, Zimbabwe is shown in Fig. 4. The relationship between temperature and spore production was described by a third-order polynomial regression Production of conidia from systemically infected leaves model. Maximum sporulation occurred at 22ЊC and local lesions (> 4000 conidia per cm2). No sporulation occurred Examination of sorghum leaf material with local lesions above 26ЊC or below 10ЊC over the 8-h period of and systemically infected field- and container-grown incubation. plants showed that container-grown plants produced 5666 conidia per cm2 (standard error ¼ 1078). Leaf Time period of release of conidia material of a similar age from the field with local lesions The time period of release of conidia from systemically produced 2437 conidia per cm2 (standard error ¼ 785) infected leaves under controlled environment conditions and systemically infected leaves from the field produced is shown in Fig. 5. The release of conidia was described 2846 conidia per cm2 (standard error ¼ 1177).

Figure 3 The frequency of detecting conidia of Peronosclerospora sorghi (W) in relation to (a) number of nights since the last rainfall, (b) mean nocturnal temperature (20·00–24·00 hours), (c) mean wind speed (20·00–24·00 hours) and (d) duration of conidia release (all data from both 1993 and 1994 growing seasons at Matopos, Zimbabwe).

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Figure 4 The effect of temperature on sporu- lation of Peronosclerospora sorghi. Sporula- tion was assessed on detached, systemically infected leaf pieces. Regression solution is a third-order polynomial model (y ¼ – 7·032x3 þ 331·68x2–4469x þ 20548, R 2 ¼ 0·89).

¹4 2 ¹1 The settling velocity of conidia (1·5 × 10 m s ) and (ra ¼ the density of air at 20ЊC (1·175 kg m¹2). The settling velocity for a conidium of P. The d of conidia as estimated using the MUI was a sorghi is calculated to be 1·5 × 10¹4 ms¹1. 23·3 mm. The results from measuring the physical diameter are similar (21·8 × 19 mm, range 14·2– 23·0 × 15·0–33·0 mm, standard error ¼ 1·72 × 2·51). Discussion The estimate of d for Lycopodium (41 mm) is similar a These data suggest that conidia of P. sorghi may play a to the observation (43·8 mm) of McCartney et al. (1993). major role in the epidemiology of SDM in the semi-arid The settling velocity can be calculated from the d of the a regions of southern Africa. The daily pattern of conidia spore (Chamberlain, 1975; McCartney et al., 1993): release was similar to that described in India and Israel (Kenneth, 1970; Shenoi & Ramalingham, 1979). The Vs ¼ðdagrÞ=ð18nraÞ; time at which conidia were released was typical, where da ¼ the aerodynamic diameter of conidia although the maximum time period over which they estimated from the MUI, g ¼ rate of fall due to gravity were detected was greater (9 h) than previously reported (9·81 m s¹1), r ¼ density of the spore (assumed to be (6 h, Shenoi & Ramalingham, 1979). In the laboratory, 1·0 × 103 kg m¹2), (n ¼ kinematic viscosity of air conidia were deposited for up to 3 h, which is similar to

Figure 5 Time period of Peronosclerospora sorghi conidia release from systemically infected leaves incubated at 22ЊC. Regres- sion solution is a third-order polynomial model (y ¼ –0·0011x3 –0·909x2 þ 15·76x – 64·072, R 2 ¼ 0·88).

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the period of deposition observed by Safeeulla (1976). In 4000 conidia per cm2 of lesion. It remains possible that the field, conditions are less uniform and this would different isolates of P. sorghi vary in their ability to result in staggered and, overall, prolonged sporulation produce conidia. on plants. Conidia cease to be detected after 08·00 h, and Duck et al. (1987) assessed the risk of the graminac- although the lesion may produce further crops of conidia eous downy mildews to other crops by their capacity for when conditions are favourable, only a single event takes asexual sporulation. They observed that P. sorghi had a place each day. Perhaps the periodic conidia production comparatively low sporulation ability compared with is dependent on the food reserves built up in the previous both P. philippinensis and P. maydis. However, it is light period, or an innate periodicity within the fungus worth noting that neither P. philippinensis nor P. maydis itself. produce local lesion infections (Williams, 1984). Local Previous work illustrated the importance of relative lesions possibly contribute significantly to the epidemic humidity and leaf wetness for conidia production of P. potential of P. sorghi within an infected sorghum crop. sorghi (Kenneth, 1970; Shenoi & Ramalingham, 1979), Local lesion infections can cover a significant proportion when temperature was adequate for spore production. of the leaf area within an infected crop which may have In this study conidia were generally produced during the only a very low incidence of systemic infection (Cohen first few nights after rain, when relative humidity would & Sherman, 1977). This may become even more be greatest. Unfortunately data on relative humidity important, considering the short latent period of local were not available at the site, but the relationship lesions (about 8 days, Cohen & Sherman, 1977; Rajasab between rainfall, temperature and wind speed indicate et al., 1980). Within the crops from which conidia were that the mean temperature (12–22ЊC) and wind condi- detected in this study, there was a rapid increase in tions (mostly < 2·5 m s¹1) that supported spore produc- incidence of local lesion infection, except in 1994 when tion would also be likely to support high relative severe drought conditions later in the season resulted in a humidity within the crop. Conidia were not produced reduction in disease. It is likely that local lesions were a at night during prolonged periods of dry weather. This major source of the conidia detected in these studies, at was especially noticeable during the dry, latter part of least until the incidence of systemic infection had risen to the 1993/4 season when there were few occasions when a significant level. conidia were detected. It is not known whether the release of conidia is active The results of the controlled environment laboratory or passive. Kenneth (1970) suggested that conidia were investigations presented in this study indicate that actively released as in other downy mildews, but no sporulation of SDM from Zimbabwe could take place studies have been undertaken to confirm this. Once the between 10 and 26ЊC (optimally at 22ЊC). In the field conidia have left the leaf surface they enter air currents. conidia were most frequently detected when the mean A combination of the settling velocity of the spore and temperature was between 15 and 19ЊC. Much of the the nature of the air currents then determines dispersal process of conidiophore development and conidia (Chamberlain, 1975; McCartney et al., 1993). The production may take place at a temperature greater settling velocity of 1·5 × 10¹4 ms¹1 for conidia of P. than 15ЊC as the mean temperature reflects the range sorghi suggests that they can be readily dispersed by between the maximum and minimum temperatures lower wind speeds at night, and supports the observa- during the entire day. tions of Rajasab et al. (1979), who detected conidia Conditions of temperature and moisture that support some distance from an infected crop. conidia production have been shown to be conducive to Information on the aerobiology of the asexual phase conidia survival, germination and infection (Bonde et al., of P. sorghi in a particular area can aid in disease 1985). Although the effects of temperature and moisture management decisions. In Zimbabwe conditions early in on infection were not investigated in this study, the the season are favourable for conidia production and observations on the production of conidia suggest that infection (Bonde et al., 1985), and planting in many conditions in these semi-arid regions of southern Africa areas may span a period of at least a few weeks. Ratoon are also likely to be suitable for conidial infection of cropping should be avoided and the risk of systemic susceptible hosts. Further work is needed to confirm that SDM infection, and yield loss may be minimized by healthy plants are infected under natural conditions in planting susceptible crops early in the season, when air- Zimbabwe. borne inoculum from early infected plants is likely to be The production of conidia of P. sorghi from systemi- lowest. cally infected sorghum leaf material is in agreement with previous observations (Futrell & Bain, 1968; Duck et al., Acknowledgements 1987), although less than that reported in India, where in excess of 10 000 conidia per cm2 was reported (Shetty This paper was written as part of a project funded by the & Safeeulla, 1981). Different varieties of sorghum may Associate Professional Officers Scheme of the Overseas affect the ability of P. sorghi to produce conidia. Development Administration, UK. Facilities were kindly However, in Uganda, Bigirwa et al. (1997) found no provided by the Natural Resources Institute and the effect of host on conidia production when comparing Institute for Arable Crops Research, Rothamsted, in the , sorghum and maize. All produced about UK and SADC/ICRISAT/SMIP in Zimbabwe.

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