ATTRA Question of the Week

E.B.
Wisconsion

Answer: I am pleased to provide you information on tomato diseases; in particular, late blight and blossom end rot.

Late blight is caused by the fungus Phytophthora infestans. A fact sheet from Ohio State University is referenced below and provides a succinct description of late blight in potatoes and tomatoes. The essential points are that late blight is spread through overwintering inoculum such as tomato vines and leaf debris, through transplants, and through climate related conditions. Thus, sanitation is an important first step. Secondly, when weather conditions are prime for late blight development a protective spray is needed during the growing season.

Spray mixtures of hydrogen peroxide, copper sulfate, and foliar fertilization are fairly effective in controlling late blight. In fact, these kinds of mixtures are what many other organic growers are using. However, there is a lack of efficacy data on what combinations of foliar materials are effective for tomato diseases. Farm-made biological teas and foliar sprays include Compost Teas (CT) and Effective Microorganisms (EM). Compost teas and EM influence the biologically active zones surrounding the plant leaf (phyllosphere) and root surfaces (rhizosphere). The beneficial microorganisms in CT and EM compete with pathogenic organisms that cause plant disease problems, and through various biocontrol mechanisms. ATTRA has additional information on these two topics, available on request.

Serenade, a biocontrol product containing Bacillus subtilis (QST 713 Strain), is manufactured by AgraQuest, Inc. in Davis, California. Serenade is labeled for tomatoes, and it has the late blight pathogen, Phytophthora infestans, listed as a target pest.

ATTRA has two publications that deal with late blight. The first publication is titled Organic Alternatives for Late Blight Control in Potatoes. The second publication is titled Organic Tomato Production and contains information on late blight as well as blossom end rot. Also see Late Blight Resources from ATTRA.

Resources:

Rowe, Randall, Sally Miller, and Richard Reidel. 1995. Late Blight of Potato and Tomato. The Ohio State University Extension Fact Sheet. Columbus, Ohio: The Ohio State University.