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Natural Pest Control in High Tunnel Tomatoes

The Tomato Magazine
June 2006

In a recent report*, the USDA documents that over the last decade off-season production of tomatoes in North America has grown by over 600 percent. Here in New York’s scenic Finger Lakes district, greenhouses and unheated high tunnels have become a profitable part of diversified vegetable farms interspersed among the dairies and vineyards.

For many vegetable farmers, growing tomatoes under plastic means a new framework for pest and disease management. It’s good-bye to early and late blight and hello to spider mites and leaf mold. These pests are rarely a problem in Northeast field tomatoes, but they take advantage of the enclosed greenhouse/tunnel environment and challenge conventional approaches to pest management.

The two-spotted spider mite decreases yields in more tomato greenhouses than any other pest. Mites thrive in the warm, dry enclosed environment. They suck juice from tomato leaves, uninhibited by natural, predatory insects, which are excluded by the plastic covering.

Miticide applications are difficult to schedule as they often have extended pre-harvest intervals (PHI). Most growers I work with harvest at least three times a week. A nine-day PHI is out of the question at the peak of the season.

Fortunately, mites can be controlled with a zero-day PHI product: natural enemies. These can be purchased from a number of sources and are shipped live to the grower. One of the most effective species is Phytoseiulus persimilis. As part of a high tunnel pepper trial we introduced ‘persimilis’ to control spider mites and were pleased with the results.

On the disease side, leaf mold, caused by Cladosporium fulvum, hits Northeast greenhouse/tunnel tomato growers hard. This disease sporulates on the underside of the leaf which makes chemical control challenging. It’s tough to get a high pressure boom sprayer inside a 20-foot wide tunnel. A very persistent disease, it is an annual occurrence in some houses.

Varieties Big Beef and Boa were significantly more susceptible to Cladosporium Leaf Mold than Geronimo and Blitz (Table 1). Resistance to disease eliminated the need for fungicide applications, increasing return per tunnel by decreasing labor and chemical inputs. Average yield per plant ranked Geronimo and Big Beef in the highest group (Chart 1).

Although Big Beef is a prolific indeterminate popular for field production, the heat and high light conditions typical of the 2005 high tunnel environment caused it to grade low with a high percentage of yellow shoulders.

Aside from resistant varieties, growers have some cultural options to reduce the chance for leaf mold. Most sources claim a relative humidity (RH) of 85 percent is required for C. fulvum to sporulate. Using RH for ventilation thresholds, leaf moisture can be kept low. The fungus grows readily in optimum tomato temperatures, but remember that colder air has lower RH than warm air.

Mold spores survive on tomato debris. Removal of all plants and cull tomatoes from the greenhouse/tunnel at the end of the season is essential. If growing directly in soil, thorough tillage is a good idea to promote further decomposition. The new season should begin with a spray down of the growing area with a labeled sterilizing product.

A versatile microorganism, C. fulvum has overcome plant resistance in the past and may do so again. Combining resistance with cultural controls will keep top varieties viable longer.

High tunnel- and greenhouse-grown tomatoes are now important to many Northeast retail produce operations. The earliness allows for market capture. When customers come for strawberries and find local tomatoes at the same time, they keep coming back all season. Increased heat units let us select varieties that would not make it in the field. And although we encounter different pests, we can control them in a natural fashion, and then market a premium product.

Editor’s note: Judson Reid is an extension associate with the Cornell Vegetable Program, Penn Yan, N.Y. He can be reached at jer11@cornell.edu.

*Cook, R and Calvin, L. 2005. Greenhouse Tomatoes Change the Dynamics of the North American Fresh Tomato Industry. Economic Research Report No. (ERR2) 86 pp, April 2005

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