Reports have come in from western, central, and southeastern Iowa concerning plantings of muskmelons and watermelons that have wilted and died shortly after being seeded or transplanted to the field. The culprit appears to be damping-off, a soilborne disease caused primarily by Pythium fungi that thrive during wet, cool weather.
Pythium damping-off attacks the roots and stems of germinating seeds and young seedlings. Stems turn brownish and water-soaked at their bases and may fall over and wither. Leaves of some transplants may turn yellow, while other plants dry up and die within a few days after planting. Dig up the root systems and you will notice discolored and/or "rat-tailed" roots; the latter term refers to roots from which the outer cortex has been stripped, leaving only the skinny white core.
Last week's carnage in melon fields appears to have been caused by an unusual combination of weather events. The coldest May in 60 years kept soil temperatures unusually low for late May. Then came a persistent wet, cool period starting May 25. Transplants that went into the ground the previous week were vulnerable to Pythium damping-off both because of the cold and the wet soil.
Most commercial melon seeds are treated with fungicides to deter damping-off. When conditions are as favorable for the disease as they were last week, however, even these fungicides aren't enough. Nevertheless, use fungicide-treated seed to give some measure of protection. Check with your supplier to determine whether your seed was fungicide-treated and the name of the fungicide used. If no treatment has been done, your seed can be treated with Captan or Thiram at the rate of about 0.5 tsp. per lb seed.
Fungicides containing the active ingredient metalaxyl can be applied at or before seeding in the greenhouse (Subdue 2E) or the field (Ridomil 2E) at labeled rates against Pythium. Does it make sense to take this precaution? If the soil temperature is unusually low and wet weather is predicted following seeding, it may make sense. Seed treatment is likely to be less expensive, however.
Replanting into damped-off melon fields generally poses no problem. The unique scenario that caused last week's melon massacre has been erased by rapidly rising soil temperatures. Therefore, seeding or transplanting into pre-existing beds should encounter minimal risk of damping-off.
This article originally appeared in the June 6, 1997 issue, p. 86.
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