Barbara Shew, Department of Plant Pathology, NC State University
The North Carolina peanut leaf spot advisory is a cooperative effort by the State Climate Office of North Carolina and the Department of Plant Pathology at NC State University. The advisory is a safe way to minimize fungicide applications by spraying only when weather conditions favor disease.
In well-rotated fields, the first fungicide spray should be applied at the very early pod stage (R3), which usually occurs in the first week of July. After the first spray, apply fungicides according to the leaf spot advisory.
Each day’s advisory contains several lines of information for each location.
Each day’s advisory: “spray today” or “do not spray today” can be found on the 7th line.
This is all you really need to know for your location.
The other lines give additional information that you may find useful.
line 1- Date and name of station. ECONET stations are indicated by an abbreviated name; airport stations are indicated by call letters. In general, ECONET stations are more reliable than airport stations. It is a good idea to check the two stations nearest you.
line 2 - Station location.
line 3 – Set date. This is used to calculate the advisory.
line 4 – Lethal conditions. A temperature of 99°F or higher for 5 straight hours OR humidity less than 40% for 8 straight hours will kill the pathogen. If lethal conditions = true, favorable hours (below) are reset to 0.
line 5 - Favorable hours. An hour is favorable for leaf spot development when the humidity is at least 95% and temperature is between 61°F and 90°F during that hour. A spray is advised when there have been at least 48 favorable hours since the set date.
line 6 – LESD (Last Effective Spray Date). A fungicide spray is assumed to protect for 14 days. You do not need to spray if you have sprayed since the LESD even when the advisory says “spray today.”
line 7 – Today’s advisory. If the advisory is “spray today” conditions are favorable for leaf spot and you should spray if no fungicide has been applied in the past 14 days. If the advisory is “do not spray today” a spray is not required.
lines 8 & 9. Growing degree days for peanuts (base 56) since the LESD and since May 1.
line 10 – Records count. The number of hourly weather observations out of the total possible observations. The advisory may not be reliable if there are a lot of missing records.
line 11 – Most recent hourly observation. This should be be 7:00:00 (7 a.m.) on the date of the advisory.