Our reports track accumulating Disease Severity Values (DSVs) for Late blight development and calculating P-days for initiating the first early blight fungicide application.

The first late blight fungicide application is recommended once 18 DSVs accumulate from green row. Green row typically occurs around the first week in May in southern New Jersey. An early season application of a protectant fungicide such as mancozeb (Dithane, Manzate, Penncozeb) or Bravo (chlorothalonil) as soon the field is accessible is suggested. Please be vigilant and keep a look out for suspect late blight infections on young plants.

Remember the threshold for P-days is 300! Once 300 P-days is reached for your location early blight fungicide applications should be initiated.

Disease severity values (DSVs) for early blight, septoria leaf spot, and tomato anthracnose development are determined daily based on leaf wetness (due to rainfall, dew) and air temperature.

On a daily basis DSV values can range from 0 to 4 where 0 = no chance for disease development to 4 = high chance for disease development. DSVs accumulate daily during the production season.

Fungicide applications are based on an individually determined DSV threshold. The first fungicide application for the control of these three diseases is not warranted until 35 DSVs have accumulated from your transplanting date. After that, growers can base fungicide applications on different DSV thresholds ranging anywhere between 15 (more conservative) and 25 DSVs (less conservative). Growers can compare accumulating DSVs to their standard weekly (7 day) fungicide maintenance programs.

For example, if 15 DSVs or more have accumulated between the time of the last fungicide application and the next regularly scheduled weekly fungicide application, recognize that conditions have been more conducive for disease development and a tighter fungicide program might be followed. If less than 15 DSVs have accumulated in the last 7 days since the last fungicide application, then conditions have not been as ideal for disease development and the next fungicide application could be stretched out until 15 DSV threshold is reached.