Diseases of Tropical Perennial Crops: Challenging Problems in Diverse Environments
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R. C. Ploetz University of Florida, Tropical Research and Education Center, Homestead Diseases of Tropical Perennial Crops: Challenging Problems in Diverse Environments The world’s oldest ecosystems are found practices, regardless of the host and envi- boldt (63) discussed the relationship two in the tropics. They are diverse, highly ronment in which it is grown. These in- centuries ago, and Darwin (26) wrote evolved, but barely understood. Diseases clude the avoidance, exclusion, and eradi- about it in his famous book. This increase that impact crops in these regions can be cation of the causal agents. Host protection in diversity is most pronounced in tropical significant contraints to production, is of great importance, as is the identifica- rain forests, which are thought to host 50% especially when they occur in lowland tion and incorporation of resistance in the of all species but occupy only 7% of the environments with high rainfall and host plant. All of these approaches are world’s landmass (162). And it appears to uniform, warm temperatures; respites from discussed with tropical perennial exam- be a general rule on our planet since it is disease pressure there are often infrequent. ples. found in the fossil record and re-estab- Difficulties in managing diseases in the lishes after mass extinctions (67). humid tropics are multiplied when the Agriculture Begins Plants are among the most prominent or- affected crops are perennial. The favorable Agriculture began after the Pleistocene ganisms that conform to the LDG. Thus, it conditions for disease development and the (last ice age) and started independently in is not surprising that most of the early presence of susceptible host tissue over several different regions (Table 1). It devel- agricultural hubs (nine of the above 12) long periods make diseases of tropical oped first in the Near East (sites in the and first crop domestications occurred in perennial crops serious management chal- Fertile Crescent and in present-day Israel the tropics, i.e., between the Tropics of lenges. and Turkey) due to a fortuitous combina- Cancer and Capricorn (Table 1). More than This topic is introduced with a few con- tion of suitable climate and useful plants half the crops in Table 2, 69 of 126 (55%), cepts on the occurrence and development and animals that could be domesticated originated in the tropics. Some tropical of these pathosystems. Peculiar aspects (33,76,144). These first farmers appeared annuals, e.g., rice, potato, and maize, are and scenarios that influence the types of at least 11,000 years ago, and were fol- now also grown in temperate zones during and extent to which different diseases lowed in quick succession by others in the summer. But essentially all tropical develop are summarized. Measures that are Northern and Southern China, Meso- perennials are restricted to the tropics due useful on annual or short-term crops may america, New Guinea, the Andes, and the to their cold sensitivity. be ineffective against these diseases. They Eastern United States (32–34). Additional A wide range of habitats is found in the are scientifically interesting problems. areas of independent development may tropics, including humid lowlands, deserts, New vectors, as for mango malformation, also include Amazonia, Ethiopia, the Sa- seasonally dry forests, grasslands, savan- or pathogens, as for bunchy top of papaya, hel, Southeast Asia, and Western Africa. nahs, montane environments, and swamps are associated with some of the diseases. During agriculture’s brief history, hu- (148,161). Further diversity in each of And some of the diseases are caused by mans have utilized numerous plants these habitats results from variable ed- two or more distinct taxa; for example, (12,21,91,135,148). At least 3,000 taxa aphic, meterologic, and biotic conditions. citrus greening, mango malformation, have been used for food and several hun- This vast array of environments enables an Panama disease, and tracheomycosis of dred more have been used for other pur- equally wide range of plants to be grown; coffee. Some of the most important poses. In Table 2, the following categories almost every crop in Table 2 can be grown diseases are host-specific and are caused have been considered: beverage, drug, somewhere in the tropical world. For ex- by either coevolved or new-encounter elastomer, fiber, food, insecticide, oil, ample, important temperate domesticates pathogens. Resistance, the most effective spice, and timber and pulp. are grown in the lowland tropics (members tool with which many of these diseases are Despite the large numbers of useful spe- of the Brassicaceae and Fabaceae are espe- managed, is usually available in coevolved cies, only a subset is very significant and cially common) and at high elevations pathosystems but may be uncommon in few are of major importance (114,128,136, where moderate temperatures exist (mem- new-encounter situations. Inadequate host 143,148). Scarcely more than a hundred bers of the Fabaceae, Poaceae, and resistance can be a significant barrier in species enter world commerce, and among Rosaceae are most notable) (114). the management of both coevolved and the food crops, few are staples: About Thurston’s (148) estimate that twice as new encounter diseases. 0.5% of the food species supply more than many crops are grown in the tropics as in General tactics are described that are 90% of the world’s food (42,148). the temperate zones of the world is proba- useful against diseases of tropical bly accurate. perennials. The successful management of Biological Diversity Studies that compare tropical and tem- plant disease utilizes several principles and in the Tropics perate ecosystems are uncommon, and a Biological diversity increases with de- disproportionate amount of the research on creasing latitude (61,67,162). This trend, Corresponding author: R. C. Ploetz, University of microorganisms has been conducted in Florida, Department of Plant Pathology, Tropical called the Latitudinal Diversity Gradient temperate zones. For example, in review- Research and Education Center, 18905 S.W. 280th (LDG), has been observed for a wide range ing the literature on fungi and bacteria in Street, Homestead, FL 33031-3314, USA; E-mail: of trophic levels and life forms. In general, forest ecosystems since 1963, Lodge et al. [email protected] species numbers increase dramatically as (87) found only 96 references for tropical one moves from the poles to the equator. forests, but 2,411 for temperate forests. doi:10.1094/ PDIS-91-6-0644 The LDG is one of the oldest recognized Despite this disparity, the LDG is also © 2007 The American Phytopathological Society patterns in the biological sciences. Hum- evident among microbes. 644 Plant Disease / Vol. 91 No. 6 Three groups of nonpathogenic fungi, temperate climates; comparatively little inoculum buildup and epidemic disease decomposers (86), endophytes (6), and plant pathological research has been con- development. arbuscular mycorrhizae (64), are very di- ducted in the developing tropical world Managing the large reservoirs of inocu- verse in the tropics, as are fungi in general (143). Work in the tropics has made sig- lum and high disease pressures that de- (156,157). Plant pathogens also appear to nificant contributions to the discipline of velop in perennial monocultures can be be more numerous and diverse in the trop- plant pathology (100,132), but much more difficult and costly. For example, manage- ics. One group, the flagellated protozoa would be revealed if resources that ap- ment of black Sigatoka leaf spot of banana (Phytomonas spp.), is rare outside the proached those used in temperate zones (black leaf streak), caused by Mycosphae- tropics (2), and 60% of the described vi- were devoted to research in the tropics. rella fijiensis, contributes as much as 25% roid species have tropical, natural hosts Perennial crops: Challenging hosts for of the final retail cost of export bananas (55). disease managers. When one considers and can fail during periods of high rainfall If one considers diseases of crop plants, the total areas planted and annual yields, or less than adequate fungicide applica- there may be an even greater difference the most important food crops are annuals. tions (105). In India, 10% of the total costs between temperate and tropical areas. Other than sugarcane (its total represents of coffee production went toward the con- Wellman (161) found a pronounced tem- harvested cane, not a final product), only trol of rust (130). And eradication efforts perate/tropical bias among the crops that production figures for maize, rice, and can be very expensive. Cacao swollen were well represented in both zones: wheat exceed 500 million metric tons per shoot, caused by Cacao swollen shoot pumpkin and squash, 19 temperate dis- year (Table 2). Although they are minor virus, in West Africa and citrus canker, eases and 111 tropical; sweet potato, components of most natural floras, annuals caused by Xanthomonas axonopodis pv. 15/187; tomato, 32/278; common bean, predominate in agriculture for the follow- citri, in Florida are worst-case examples of 52/253+; and potato, 91/175. Wellman ing reasons: they produce quick results where large sums of money were invested (159–161) concluded that for every disease after planting; when stored, they enable in ultimately unsuccessful campaigns. that occurred on a given crop in temperate escape from unfavorable climatic condi- Due to long-term selection pressure, areas there were 10 in the tropics.