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
© 2006 Columbia
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