Management of Phytophthora Blight of Cucurbit Crops in Illinois

M. Babadoost

Universtiy of Illinois

Illinois Cucurbit Industry. Illinois produces approximately 30,000 acres of cucurbit crops, including 12,000 acres jack-o-lantern pumpkins, 10,000 acres processing pumpkins, 2,500 acres squash, 2,000 acres cucumber, 2,000 acres watermelon, and 1,500 acres cantaloupes and muskmelons. Due to implementation of effective horticultural practices and disease management strategies, acreage of processing pumpkins in Illinois in 2002 increased by 65% compared to that of 2000. Approximately 90% of processing pumpkins produced in the United States are grown in Illinois.

Phytophthora Blight. Phytophthora blight, caused by Phytophthora capsici, has become one of the most serious threats to production and industry of cucurbit crops in Illinois. Also, P. capsici infects bean, beet, carrot, eggplant, okra, onion, pepper, tomato, and several weed species in cucurbit fields. In the past four years, Phytophthora blight has been epidemic in Illinois, causing up to 100% yield losses.

P. capsici is a soilborne pathogen and is also disseminated by air currents. It survives in the soil indefinitely. P. capsici can strike host plants at any time from planting through harvest. Symptoms produced by this pathogen include pre- and post-emergence damping-off, root rot, crown rot, stem lesions, leaf spots, foliar blight, and fruit rot. Until 2001, there was no method available to provide adequate protection of vegetables against P. capsici.

Disease Management. Three methods were developed to manage Phytophthora blight on cucurbit crops, which are: (i) seed treatment, (ii) foliar fungicide spray, and (iii) red-light treatment.

Seed treatment. Seed treatment with mefenoxam (0.64 fl. oz/100 lb seed) and metalaxyl (1.5 fl. oz/100 lb seed) significantly reduced pre- and post-emergence damping-off of seedlings caused by P. capsici in pumpkins. In greenhouse studies, 31 days after seeding, at inoculum level 600 cfu/g soil, the average seedling stands were 88.3, 85.8, and 45.7%, for Apron, Allegiance, and untreated control, respectively. In field trials, the average seedling stands 35 days after seeding were 76.7, 74.7, and 44.9% for Apron, Allegiance, and untreated control, respectively. Seed treatment with Apron or Allegiance did not have any significant effect on either seed germination or seedling vigor.

Foliar Fungicide Spray. Five application of dimethomorph (6.4 oz Acrobat 50WP/acre) plus copper sulfate (2.5 lb Cuprofix Disperss/acre), at a weekly schedule, provided effective protection against foliar blight and fruit rot, caused by P. capsici, in pumpkin fields. Application of the fungicides was begun at the first sign of the disease. Fungicide spray reduced yield losses from 100% to less than 10% in the commercial fields.

Red-Light Treatment. A study was conducted in the greenhouse to investigate the effects of red light (600-700 nm) on the subsequent occurrence of seedling infection of pumpkin caused by P. capsici. Four-week-old seedlings were inoculated with zoospores or transplanted into pots filled with artificially infested soil mix. Only 21% of the seedlings treated with red light became infected. In comparison, 78% of control seedlings, either grown under white light or under natural daylight, were infected and died. Investigations are underway to evaluate the effectiveness of red-light induced resistance on controlling Phytophthora blight in commercial fields of cucurbit crops.

IPM of Phytophthora Blight. Seed treatment (Apron or Allegiance) along with foliar spray of

dimethomorph (Acrobat) provided effective protection to pumpkin plants against P. capsici in

commercial fields. Investigations are underway to develop effective methods of integration of

seed treatment, red-light treatment, and foliar application of fungicides for management of Phytophthora infection on cucurbit crops.