- Fruits: melons
- Vegetables: beans, beets, broccoli, brussel sprouts, cabbages, carrots, cauliflower, celery, cucurbits, eggplant, greens (leafy), lentils, onions, parsnips, peas (culinary), peppers, rutabagas, sweet corn, tomatoes, turnips
- Crop Production: crop rotation
- Pest Management: genetic resistance, prevention, general pest management
Forty-five species of crop and weed plants were screened for their susceptibility to P. capsici. Twenty-two crop species succumbed to the disease, 14 did not. Pathogenicity tests in the greenhouse showed that Phytophthora capsici isolates were significantly less virulent on eggplant than they were on cucurbits, pepper, and tomato. Molecular study showed that there are significant differences in pathogenicity and genetics among isolates of P. capsici. The results of this research will help in establishing effective cropping rotations for management of P. capsici in vegetable and weed control, thus establishing sustainable vegetable, particularly cucurbit, production.
Illinois farmers grow approximately 12,000 acres of jack-o-lantern pumpkins and 10,000 acres of processing pumpkins each year (3). About 90% of commercial processing pumpkins produced in the United State (US) are grown in Illinois. Also about 10,000 acres of cantaloupe, cucumbers, eggplants, pepper, tomatoes, and watermelon are grown in Illinois each year. Phytophthora blight, caused by Phytophthora capsici Leonian, has become one of the most serious threats to production of cucurbits, eggplants, and peppers in Illinois, as well as nationwide (3,5,8,23,24). Phytophthora blight causes up to 100% yield losses in commercial fields of the above-mentioned crops in Illinois (2,3). Heavy crop losses of cucurbits, particularly pumpkins, to Phytophthora blight, lack of resistant cultivars, and inadequate effect of chemicals in controlling the disease prompted our investigation to develop strategies to manage the disease and minimize crop losses. Crop rotation to minimize primary inoculum of P. capsici in infested fields is an important component of disease management strategies.
Phytophthora capsici is a soil-borne Oomycete and survives as oospores in soil for several years (5,29). The pathogen can infect all parts of the plant at any growth stage, causing seedling death, crown rot, leaf spot, foliar blight, and fruit rot (3,5,29). Infection of the foliage occurs when zoospores of P. capsici are splashed onto the plant surfaces from soil during rainfall or irrigation (16). Propagules of P. capsici are dispersed by water, soil, and air currents (22). Seedling death occurs in wet and warm (20 to 30C) soil conditions (5,11). Therefore, screening seedlings for their susceptibility to P. capsici in a greenhouse is a reliable approach for determining host range of P. capsici and disease resistance.
Forty-nine plant species have been reported infected with P. capsici (3). Among the major hosts of P. capsici are peppers (Capsicum annuum), watermelon (Citrullum lanatus), cantaloupe (Cucumis melo), honeydew melon (C. melo), cucumber (Cucumis sativus), blue Hubbard squash (Cucurbita maxima), acorn squash (Cucurbita moschata), gourd (C. moschata), processing pumpkin (C. moschata), yellow squash (C. pepo), zucchini squash (C. pepo), tomato (Lycopersicon esculentum), black pepper (Piper nigrum), and eggplant (Solanum melongena).
Cucurbit isolates of P. capsici have been reported pathogenic on cucurbits, pepper, and tomato (13). Polach and Wenster (20) reported distinct pathogenic strains identified among isolates of P. capsici from tomato, pepper and squash. Ristaino (23) evaluated the relative virulence of isolates of P. capsici from cucurbits (cucumber and squash) on pepper and found differences in virulence among the isolates. In Italy, Tamietti and Valentino (24) grouped P. capsici isolates into 13 classes depending on their ability to infect different plant species (pepper, tomato, eggplant, melon, squash, pea, and French bean). In South Korea, Lee et al. (16) studied aggressiveness of P. capsici isolates from pepper and pumpkin on pumpkin cultivars and reported significant pathogen-host interactions.
Visual observations of symptoms and isolation of the pathogen from infected tissue have been the methods employed for diagnosing diseases caused by P. capsici (3,17). However, this approach is laborious and time consuming. Therefore, it was necessary to develop a rapid and sensitive diagnostic method for detection of P. capsici in plant tissue. The polymerase chain reaction (PCR) assay is an approach that allows for the rapid detection of Phytophthora species in plants (16,22,24,25).
Genetic diversity is common among isolates of fungal species (14,27). Different methods have been used to study the genetic variation of fungi (8,22,24,28). Internal transcribed spacers (ITS) regions have been used to determine genetic differences among species of Phytophthora, as well as other fungi (22,28). Inter-simple sequence repeats (ISSR) amplification is a new technique that could rapidly differentiate closely related individuals within a fungal species (28). Amplified fragment-length polymorphism (AFLP) is a recently developed polymerase chain reaction (PCR) that provides genetic markers for fingerprinting, mapping, and studying genetic relationships among populations within fungal species (12,21). Alonoso & Glenn (1) modified the original AFLP and provided an easy protocol for digestion of genomic DNA and ligation with adapters in one reaction.
Genetic variation among P. capsici isolates has been reported in some vegetable growing areas in the world (9,18). Our research reports the results of investigation on pathogenic and genetic variation among P. capsici isolates on eggplant, pepper, pumpkin, squash, tomato, and watermelon in Illinois.
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The main objective of this study was to determine pathogenic diversity of P. capsici isolates for establishing sustainable pumpkin production in Illinois. The specific objectives of this research were:
1. Determine the susceptibility of crops grown in rotation with cucurbits and weeds that commonly grow in cucurbit fields to P. capsici.
2. Assess the virulence of P. capsici isolates on different pumpkin cultivars.
3. Determine genetic and pathogenicity variation among isolates of P. capsici from Illinois.