Left untreated, bacterial and fungal diseases can reduce tomato yields by as much as 70 percent. Managed with a spray schedule using multiple crop protection chemicals, tomato producers can almost triple their output and at the same time maintain the effectiveness of the products.
No single fungicide will control all the different foliar diseases. That's why it's important to use a program like the one developed at North Carolina State University's Mountain Horticultural Crops Research Station in Fletcher.
“The program is designed to manage early blight, late blight, bacterial speck and bacterial canker,” says Paul Shoemaker, North Carolina State Extension plant pathologist.
The spray program spreads the risk among several fungicides in an effort to avoid resistance. For example, mancozeb products are used early in the season, prior to harvest and chlorothalonil once harvest begins. Shoemaker also recommends alternating treatments with Quadris. Producers also use cooper for bacterial canker. Actigard, which was registered last season, is recommended every two weeks prior to harvest for bacterial speck and bacterial spot. Most strains of bacterial speck are resistant to copper, Shoemaker says.
“This spray program is designed to slow down the resistance of the pathogen to the materials,” Shoemaker told a group of producers at the Southeast Fruit and Vegetable Expo held in Greensboro, N.C.
The North Carolina State Extension plant pathologist talked about early and late blight, bacterial speck, bacterial canker, and bacterial spot.
Early and late blight are more of a problem at higher elevations than they are elsewhere, Shoemaker says. A fungus which over-winters on tomato debris in the soil causes early blight. If left untreated, it develops slowly and would take about eight weeks or more to defoliate a significant number of leaves from the tomato plants. “But it can shorten the production season if it's not controlled.”
Late blight comes from sources outside the growing area. It finds favorable conditions during cloudy weather, Shoemaker says. “If you're not on a good spray program once it's introduced, late blight can cause considerable damage in a very short time.” Chlorothalonil products are effective when used preventatively, before blight becomes a problem.
Bacterial spot, canker and speck all can be seed borne. These diseases often appear three to five weeks after planting, developing undetected on tomato plants for several weeks. Bacterial diseases first develop on the leaves, but can also cause spotting on the fruit, Shoemaker's research shows.
“Bacterial canker can go systemic in the plant and cause the plant to wilt. “The best control for bacterial diseases is to be on a preventative spray program during transplant production with copper or streptomycin in the greenhouse,” Shoemaker says.
In the field, copper can be used for bacterial canker. Actigard, which turns on the plants defense mechanisms, is effective against bacterial speck and spot.
The introduction of Quadris into the spray schedule has lengthened the growing season for tomato producers, Shoemaker says.
“Quadris has also allowed us to extend the spray schedule to seven days from five days,” he says. The plant pathologist used to recommend sprays every five days. “If conditions become wet, and late blight is a problem, the producer may want to tighten it up to a five-day schedule and include chlorothalonil.”
Using a spray schedule can reduce the impact of bacterial and blight diseases on tomatoes, Shoemaker says.
Alternate chemistry, such as mancozeb, chlorothalonil, copper, Quadris and Actigard can also help reduce the risk of resistance from the pathogens.
The spray program is outlined on the Web at http://fletcher.ces.state.nc.us/staff/shoemaker/