Understanding Damping Off Associated with Early Transplanting of Warm Season Vegetables

Published: May 1, 2011

Many growers try to plant warm season vegetables earlier to take advantage of high early season prices. Unfortunately, the chances of transplanting crops like tomatoes, peppers, and cucurbits into field conditions favorable to damping off is increased, even if using good practices like raised beds covered with plastic mulch and protected by floating row covers. This spring’s fluctuating temperatures (often on the cool side), along with cloudy and rainy conditions, further aggravate the situation. So what is damping off and what can you do about this situation?

Damping-off generically refers to seedling diseases from a number of fungi including species of Pythium*, Phytophthora*, Fusarium**, Rhizoctonia**, Sclerotinia**, Sclerotium**, and Botrytis**. It is often associated with germinating seeds and young seedlings, which are vulnerable to attack by these pathogens during periods of unfavorable growing conditions- damp, cool, and low light conditions.

*Oomycete
** True fungi

The best control of damping-off for early transplanted warm season vegetables is to start with disease free transplants and clean growing media. Once this disease has started in a seedling flat, it may be difficult or impossible to control or determine if ‘healthy looking plants’ are uninfected. Treating transplants before field planting should be considered in these situations:

• The soil temperatures are still cool, may stay cool, and wet/cloudy weather likely
• Damping off has occurred in your greenhouse and/or adjacent flats.

What fungicides are available to use? Unfortunately two common broad spectrum fungicides are no longer labeled for vegetables- Captan and Banrot. Synthetic products available are Terraclor (PCNB), Ridomil (mefenoxam), Previcur Flex (propamocarb ), and Ranman (cyazofamid). Ridomil and Ranman are currently labeled for cucurbits and fruiting vegetables, but Previcur Flex is only labeled for cucurbits. Ridomil (mefenoxam), propamocarb (Previcur Flex), and Ranman are effective only against Oomycetes (Pythium and Phytophthora), but will not control any of the ‘true fungi’ damping off species (see 2nd paragraph above). Terraclor can provide protection against Rhizoctonia diseases. Another option is the biologically based fungicides, which are generally considered better at preventing diseases than curing them. Streptomyces griseovirdis (Mycostop), Trichoderma virens (SoilGard), and Trichoderma harzianum (RootShield), Basillius subtillis (Companion) are biological controls that are labeled for vegetables, including organic production.

A few of good questions are:

• If a mild infection occurs, do the plants recover fully, or is there a reduction in yield potential? It depends on the crop, but it is likely yields will be less. Crucifer seedlings with a mild late infection have shown significant yield reductions. For warm season vegetables a later mild infection would likely damage the vascular system near the crown and reduce yield potential.
• Will a field applied fungicide help once the disease is diagnosed? Fungicides applied at the very first signs of the disease symptoms may help reduce/ limit the infection, but at later stages the chemicals will be ineffective. A drench should reach the same depth as the transplant plug. Application instructions will likely vary with product formulation and should be carefully followed.
• And how does one separate the complicating factor of low soil oxygen aggravating the conditions....in other words, is it more about the weather than anything ‘you’ can do? It can be difficult to determine if the seed rot/ damping-off is from disease, excess fertilizer, temperature damage (heat and cold), soil saturation, etc. Damping-off is most effectively controlled with cultural practices (most importantly water management and sanitation) than chemicals. Weather has a big impact but things that can be done to limit damping-off damage are use of raised beds and improving soil structure. Waiting to plant in the ground until the soil temperature is conducive to good plant growth and waiting a few days after a heavy rain will also help.