Andy Wyenandt, Assistant Extension Specialist in Vegetable Pathology, Rutgers University; wyenandt@aesop.rutgers.edu
With the extremely hot weather and hit or miss downpours lately, damping-off can be very common this time of year. Damping-off can kill seedlings before they break the soil line (pre-emergent damping-off) or kill seedlings soon after they emerge (post-emergent damping-off). Common pathogens that cause damping-off include Pythium, Phytophthora, Rhizoctonia and Fusarium species. Although all four pathogens are associated with damping-off, the conditions which favor their development are very different. In general, Phytophthora and Pythium are more likely to cause damping-off in cool, wet soils. While, Rhizoctonia and Fusarium are more likely to cause damping-off under warmer, drier conditions. In general, Pythium tends to kill seedlings before they emerge whereas Rhizoctonia and Fusarium tend to kill seedlings after they emerge. There are exceptions to the rule, but none the less, all damping-off pathogens can cause serious losses if not controlled properly. Control of damping-off depends on a number of factors. First, is recognizing the conditions which may be leading to the problem (i.e. weather, greenhouse growing conditions) and second, identifying the pathogen causing the problem. Why is this so important? The fungicides applied to prevent or control damping-off are specific in the pathogens they control. Fungicides used to control Pythium or Phytophthora won’t control the other damping-off pathogens.
Why is this? The biology of the fungus and the mode of action of the fungicide dictates fungicide efficacy. For example, Ridomil Gold and Ultra Flourish (mefenoxam, FRAC code 4) and Previcur Flex (propamocarb, 28) helps control the ‘water molds’ (Pythium and Phytophthora) where Terraclor (PCNB, 14), Rovral (iprodione, 2) and Quadris (azoxystrobin, 11) helps control damping-off caused by Rhizoctonia. Therefore, it is extremely important to know which pathogen is causing the damping-off problem and which fungicide to properly apply. Always refer to the fungicide label for crop use, pathogens controlled and application rates.