Of the 91 tested cucurbit seed samples from thirteen countries nine from four countries were found to be infected with Didymella bryoniae. The pathogen is located on and in the seed coat including the perisperm and in the tissue of the cotyledons. Primary seedling infection occurred on the radicle, hypoctyl and cotyledons. Infection of the radicle generally caused pre‐emergence tot while infection on the hypocotyl and cotyledons developed furthere inoculum for infection of the first true leaves and the stem. Experimentally, all the isolates of D. bryoniae could infect cucumber (Cucumis sativus), oriental melon (Cucumis melo var. makuwa), pumpkin (Cucurbita pepo) and watermelon (Citrullus vulgaris) at different growth stages; the susceptibility of cucumber and pumpkin was markedly influenced by previailing humid conditions.The blotter method was found more suitable for detection of seed‐borne infection than the agar plate method.
Out of 140 soybean samples tested from 17 countries, 42 originating from Brazil, Colombia, Iran, Korea, Malaysia, Thailand and the United States were found to be contaminated by oospores of Peronospora manshurica (Naum.) Syd. Oospores are generally present as a crust on the seed coat, and are easily detected during visual examination; however, individual spores may stick to the surface of non‐crusted seed, and these spores are detectable only by examining surface washings.
A new method for testing the viability of oospores by the tetrazolium chloride test is described. Viable oospores turn orange‐red. In all oospore collections, viability tested by this method was generally higher than germination recorded in water. One to 2‐year‐old oospores showed 30 to 39% viability, while an 8‐year‐old collection had about 20% viable spores. Oospores collected from 1‐year‐old seed treated with captan showed 6 to 11 % viability.
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