Return of rain will be the return of Cercospora leaf spot in sugarbeets
Cercospora leaf spot infections in sugarbeets will increase with the return of rainfall, high humidity and normal temperatures.
The incidence of Cercospora leaf spot in Michigan has been low through most of July due to low humidity, droughty conditions and extreme temperatures (mid 90s). Rainfall has been in short supply for most of the sugarbeet growing area, causing smaller than normal beet canopies. The combination of environmental conditions combined with well-timed fungicide spray application has made leaf spot infections hard to find. On July 26, 2012, the entire sugarbeet growing area received up to 2 inches of rainfall. Environmental conditions are now very conducive to leaf spot infections.
When conditions are favorable (warm temperatures and high humidity), spores will be produced which will be deposited onto sugarbeet leaves by wind or splashing rain. Germinating spores enter the leaf through stomata (tiny holes) in the leaf. Ideal environmental conditions for Cercospora are daytime temperatures of 75 to 90 degrees F with night temperatures above 60 degrees, coupled with wet leaves (from dew or rain) for 10 or more hours. After the leaf has become infected it will take from five days to three weeks for spots to develop, depending on environmental conditions and beet resistance.
Current Cercospora fungicides have the best efficacy when used as a preventative, before any spots are seen. This is extremely important when utilizing a contact material. Some systemic Triazole fungicides may give a slight kick-back of about a day or two. Be aware once spots are seen, fungicide application will not control existing spots. Proper disease management involves delaying the onset of disease for as long as you can.
With the return of adequate rainfall, sugarbeets will be in a significant period of growth for both the root and leaf. Any new growth that appears after fungicide applications will be left unprotected and will be vulnerable to leaf spot infections. It is important that recommended spray programs be followed including the appropriate Disease Severity Value (DSV) and fungicide tank-mixes to minimize potential leaf spot resistance. The new recommendations have also tightened up the spray intervals.
Reapplication timings for your area can be found on the BEETcast website. Control measures are normally justified until early to mid-September for beets that are slated for permanent pile.