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 Tom –

This season I seemed to have a new set of bugs and disease that were only minor problems in the past. Did you notice that too?

— Perplexed in Grapevine



Dear Perplexed –

I noticed that lettuce bacterial spot was particularly bad this year. It was an irritation in the past but it became a major problem this year. My theory is that the warm temperatures in June favored some pathogens and insects over plant resistance and natural controls. Once they became established the problem lasted most of the season.

My theory is based in part on an experience several years ago with grey mold (botrytis) in my tomato greenhouse. Botrytis infects tomato blossoms so fruit never form. It can invade the calyx so fruit drop before they are ripe. It can turn fruit with minor imperfections into culls. In consulting with other growers, they recommended keeping our greenhouse temperature above 55 degrees. I reset the greenhouse thermostat from 50 to 55, and the botrytis disappeared within a week. That disease likes growing in cool conditions but produces spores at higher temperatures (in the mid-seventies). By keeping the temperature above the ideal growth temperature for the fungal growth and then running quickly into the 80s when the sun rose, we avoided temperatures that favor both fungal growth and also fungus reproduction. Our higher production and better fruit quality was well worth the expense of extra greenhouse heat to maintain a five degree higher nighttime temperature.

Inside the greenhouse we obviously have more control over temperature than in the field. Our nightly lows in the summer are typically in the sixties but for those hot weeks in June, the overnight lows were often in the seventies. As it turns out bacterial spot in lettuce grows the best at 74 degrees. Adding the moisture of evening dew created a pathogen incubator in our lettuce patch. A disease that was an irritation in the past took down several plantings of lettuce this year.

 Without access to a weather “thermostat” for outside temperatures, we plan to be more aware of overnight lows. This graphic may be useful in tracking the projected lows. It is found at:

 If we see the nightly low leveling off at temperatures that favor our pathogen (mid-seventies), it may be time to turn up our controls a notch or two. Next year we intend to

  • plant more resistant varieties during warm periods,

  • spray regularly with immune stimulants like Regalia, and

  • physically separate plantings to make the spread of disease more difficult.

One last thought on average evening temperatures is that climate change may make extreme summer temperature more and more common. If June 2012 is a glimpse of the future, growers can consider it a “heads up” as we prepare for future growing seasons.

Thanks for your question.

— Tom

P.S. Gemplers has an ice suit for overheated growers at

Tom Elmore

Author: Tom Elmore

Tom Elmore is co-owner and operator of Thatchmore Farm in Leicester NC. He has grown certified organic fruits and vegetables for 25 years and serves on the Boards of the NC Greenhouse Vegetable Growers Association and the Organic Growers School.

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