Detection and Management of Phytophthora and Pythium in Carrot, Tomato, Cucurbits, and Asparagus

To better understand and manage vegetable diseases caused by Phytophthora and Pythium spp., studies were conducted to: 1) Develop a nested-PCR method to detect Pythium spp. responsible for carrot cavity spot, and qPCR method for Phytophthora capsici quantification from soil; 2) Examine the relationship between cucumber fruit development and resistance to P. capsici; 3) screen pepper and tomato germplasm for resistance to P. capsici; 4) Evaluate various fungicide chemistries for activity on P. capsici; 5) Develop disease suppressive soil for managing carrot cavity spot; and  6) Tested effects of cover crops on P. capsici population in cucumber and summer squash production. Methods and results are as follow. Resistant cucumber fruit [16 days post pollination (dpp)] did not prevent zoospore germination, but had fewer appressoria and more aberrant germ tubes of P. capsici. Pyrosequencing analysis of RNA from 0, 4, 8, 12, and 16 dpp fruit showed patterns of gene expression consistent with a profile of genes initially associated with cell division and rapid growth, followed by genes associated with defense. Germplasm screening showed pepper lines CM334, NY07-8007, NY07-8006, NY07-8006 were resistant to P. capsici. None of the commercial cultivars were resistant to isolate 12889. Tomato Solanum habrochaites accession LA407 was highly tolerant, and varieties Ha7998, Fla7600, Jolly Elf, and Talladega were moderately tolerant. When Zucchini ‘Justice III’ was grown in a P. capsici-infested field. Ridomil Gold MZ, Gavel, Kocide + Acrobat or Presidio or Revus or Ranman, all resulted in fewer dead or wilted plants than the untreated control. Cell packs were filled with Pythium ultimum-infested media and drenched with fungicides prior to sowing carrot ‘Napoli’. All chemicals except Ranman resulted in more emerged seedling compared to the untreated inoculated packs. Conventional, transitional, and organic farming systems were established. Soil samples from the plots were collected monthly, spiked with P. ultimum, and planted with cucumber seeds. The detrimental effects of Pythium spp. were reduced, but the beneficial effect of the elevated soil organic matter was fleeting. Both microbial activity and soil organic matter were positively correlated with disease suppression. In a commercial field infested with P. capsici oilseed radish (Raphanus sativus), oriental mustard (Brassica juncea), brown mustard (B. juncea), cereal rye (Secale cereale), and metam sodium affected soil microbial populations.  However, the brassica cover crops did not impact disease incidence. Therefore, the cover crops can be used to improve crop rotation when P. capsici control is not the primary goal.