Oomycete diseases of cucurbits: History, significance, and management

Oomycetes were originally classified among the fungi, as Oomycota. It is now known that oomycetes are allied with certain algae and thus they were excluded from true fungi and placed in the kingdom Chromista. Oomycetes produce filamentous hyphae that resemble those of the true fungi, but without cross septa (coenocytic hyphae). The cell wall in oomycetes is composed primarily of β-glucans and cellulose, while those of true fungi are mostly chitin. Their asexual reproduction is by means of sporangia and biflagellate zoospore, and their sexual spore is an oospore. Oospores are thick-walled, resistant structures capable of surviving under unfavorable environmental conditions. Major oomycete pathogens of cucurbits are Phytophthora capsici, Pseudoperonospora cubensis, and Pythium spp. Phytophthora capsici is the most important Phytophthora species infecting cucurbits and it can infect cucurbit plants at any growth stage causing damping-off, crown rot, foliar blight, and fruit rot. Phytophthora capsici was first described in 1922 in New Mexico, USA. Since then, this pathogen has been reported from many cucurbit-growing areas in the world. It can infect more than 50 plant species in more than 15 plant families. Main hosts of P. capsici are cucurbits and peppers. Phytophthora capsici is a soil-borne pathogen and survives between crops as oospores in soil or mycelium in plant debris. Oospores can survive in the soil, in the absence of a host plant, up to 4 years. Pseudoperonospora cubensis incites downy mildew, a significant disease of cucurbits. Downy mildew of cucurbits was first reported in 1868 in Cuba. Now, this disease occurs in most of the cucurbit-growing areas in the world. Pseudoperonospora cubensis can infect cucurbit plants at all ages. Infection is limited to leaves. Symptoms of downy mildew are small, slightly chlorotic to bright yellow areas on the upper leaf surface and mold (sporangiophores and sporangia) of P. cubensis on the corresponding lower leaf surface. Pseudoperonospora cubensis is an obligate parasite and thus survives only on living host tissues. Host range of P. cubensis is limited to only members of the Cucurbitaceae family. Temperatures of 15–25◦C and moist conditions are favorable for downy mildew development. Pythium spp. are soil inhabitants, existing as nonspecialized parasites. Pythium anandrum, P. aphanidermatum, P. irregular, P. myriotylum, oligandrum, P. periplocum, and P. ultimum cause damping-off, root rot, and Pythium cottony leak. Pythium spp. cause infection in a wide range of species in more than 24 pant families. Cooler and moist conditions favor infection by Pythium spp. All of the oomycete pathogens of cucurbits have been reported from cucurbit-growing areas throughout the world. Under favorable environmental conditions, these pathogens can cause up to 100% crop losses. Strategies for effective management of diseases caused by P. capsici, P. cubensis, and Pythium spp. in cucurbits are different from each other, but no single method is adequate for successful control of any of these pathogens. Successful, long-term management of each of the diseases caused by oomycete pathogens in cucurbits requires integrated strategies with measures that eradicate or reduce initial inoculum, reduce effectiveness of primary inoculum, increase the resistance of the plant, delay the onset of disease, and slow spread of disease.