P. viticola is an obligately biotrophic oomycete placed into the kingdom Chromist
P. cubensis is a species of water mould known for causing downy mildew on cucurbits such as cantaloupe, cucumber, pumpkin, squash and watermelon. This water mould is an important pathogen of all these crops, especially in areas with high humidity and rainfall. In most years the disease is an annual, late-season problem on CUCURBITS.
All green parts with stomata of the host plant are infected. On young leaves, lesions appear as translucent 'oil spots'. Oil spots become dry and necrotic, first in the centre and later throughout the entire lesion. Lesions are restricted by veins to form angular, yellow to reddish-brown spots which combine to form a patchwork. Sporulation only occurs on the lower leaf surface, where the sporangia on sporangiophores appear as a white growth. On older leaves sporulation occurs primarily on the margins of the lesion.
Infected inflorescences and young berries become yellow or gray and may be covered with cottony spores under favorable conditions. Berries infected later in the season become discolored and shrivel but do not support sporulation. This stage is sometimes referred to as the 'brown rot' phase. Potential yield losses remain high, ranging from 50 to 100% under favorable conditions.
The pathogen causes angular chlorotic lesions on the foliage. These lesions appear angular because they are bound by leaf veins. During humid conditions, inspection of the underside of the leaf reveals gray-brown to purplish-black fungal growth. This downy material is the sporulation of the pathogen. Magnification of the sporulation reveals the acutely and dichotomously branched sporangiophores bearing lemon-shaped sporangia. Eventually, leaves will turn necrotic and curl upwards. The disease is sometimes called wildfire because of how rapidly it progresses, as if the crop were burned by fire.
Symptoms on watermelon and cantaloupe are different from on other cucurbits; leaf spots are typically not angular and turn brown to black in color. Often, an exaggerated upward leaf-curling will occur. Regardless of which cucurbit is involved, only the leaves are infected, not fruit, flowers, stems or roots. Disease of the leaves results in three major effects: 1. reduced yields, 2. a greater proportion of misshapen fruit, especially in cucumber, and 3. sunscalded fruit, due to increased exposure to direct sunlight, especially in watermelon and winter squash.
Oospores are spherical, 28-40 µm in diameter, covered by two inner oospore membranes and an outer wrinkled oospore wall. They germinate in spring when temperatures reach 10°C and vineyard soils are wet. The germ tube terminates in a macrosporangium which releases an average of 8-20 and up to 60 zoospores. Zoospores require surface wetness to infect the host and infection takes place only through the stomata. Zoospores swim on the tissue surface, encyst near stomata, and each spore forms a single germ tube which penetrates the stomata. In the substomatal cavity the germ tube swells, forming a substomatal vesicle from which a single hypha arises growing intercellularly. In as little as 3.5 hours the first haustorium forms where the pathogen contacts the host cells. Later additional haustoria form parasitizing the mesophyll cells. The incubation time, the period between infection and the first appearance of symptoms, depends on temperature and ranges from 4 to 21 days, with an average of 7-10 days. The pathogen sporulates through stomata during warm, humid nights.
The sporangiophores are hypophyllous, arborescent, 130-250(-700) x 11-14 µm, branching monopodially in the upper third at right angles to the main axis, and with a base tapering to a conical point; branches in a whorl of 4-5, 35-45 µm long, often with two opposite secondary branches 15-20 µm long, all having 3-4 conical tips 10 µm long, 6 µm wide at base, diverging at right angles and tapering to a terminal swelling.
The sporangia are ovoid, colorless, 20 x 14µm, each producing 1-6 zoospores.
For sporulation, P. viticola requires at least 95-98% RH, temperatures between 10 and 30°C and at least 4 hours of darkness. Individual lesions resporulate a number of times under favorable conditions, and can retain the potential to sporulate for several months. Secondary cycles of infection occur repeatedly throughout the growing season if weather conditions are favorable.
P. cubensis is an obligate parasite or biotroph, meaning that it requires live host tissue in order to survive and reproduce. Because of this characteristic, the pathogen must overwinter in an area that does not experience a hard frost, such as southern Florida, and where wild or cultivated cucurbits are present. The spores are dispersed via wind to neighboring plants and fields and often over long distances. Symptoms appear 4 to 12 days after infection. The pathogen thrives under cool and moist conditions, but can do well under a wide range of conditions. Optimum conditions for sporulation are 15 °C with 6 to 12 hours of moisture present, often in the form of morning dew. Even when high daytime temperatures are not favorable for the pathogen (>35 °C), nighttime temperatures may be very suitable. Oospores (thick-walled, resting spores) of P. cubensis are rare and their role in nature is unknown.