Learn to recognize diseases on vegetable transplants in the greenhouse

Diseases can get an early start in the greenhouse, making control difficult in the field.

Bacterial spot on tomato transplants for field production establishment has been confirmed. This disease is one of three bacterial problems that growers should be looking for on their transplants. Both bacterial spot and bacterial speck cause lesions on the foliage of the leaves and sometimes on the petiole and stem.

Bacterial spot
Bacterial spot occurs as dark spots or blotches on
the leaves and stems of tomato.

Bacterial speck
Bacterial speck lesions on tomato leaves are smaller
and surrounded by a yellow halo.

The third bacterial problem is called bacterial canker and it looks very different from the other two tomato transplant problems. Bacterial canker does not cause dark spotting or blighting, but rather appears to have a light-brown appearance that often follows the vein pattern in the leaf and typically extends to the petiole. The symptoms of bacterial canker are often confused with cultural problems because of the relatively non-descriptive disease symptoms.

Bacterial canker seedlings
Bacterial canker starts as tan-colored, blister-like lesions that
progress to form streaking and cankering on leaves and petioles
of tomatoes.

Tomato transplants in the greenhouse can be sprayed preventively with copper (Kocide 3000 is an example) in combination with Agri-Strep 17. Because the vegetable transplants receive frequent overhead watering, the fungicide residue will be compromised and frequent sprays will be needed. A five-day spray schedule may be needed when disease threatens (overcast, wet weather) or when early symptoms are first discovered.

Downy mildew has been confirmed on cabbage transplants grown in the greenhouse for field production. This disease can cause severe damage to seedlings. While downy mildew is generally not a significant problem during the early stages of production, it can cause big losses near maturity and rotting during transit and storage.

Downy mildew symptoms Downy mildew symptoms
Downy mildew symptoms on green- and purple-leafed cabbage transplants.

Downy mildew lesions Downy mildew lesions
Close up of downy mildew lesions on cabbage leaves.

The downy mildew is favored by wet conditions (fog, rain, drizzle or dew). There are few fungicides that are labeled for use in the greenhouse against downy mildew on cabbage and include phosphorus acid salts (Alude is an example). Greenhouses growing cabbage seedlings that are near cabbage production fields are at risk. The downy mildew can overwinter in cabbage production fields, infect newly planted cabbage seedlings, and then the downy mildew spores can move via air currents to cabbage seedlings growing in nearby greenhouses.

Cultural controls include keeping the foliage as dry as possible so that diseases are not favored. Watering at a time of day when the foliage can dry quickly is important in keeping plant pathogens limited. Good ventilation and air movement help to purge moist air and replace it with drier air. In some instances, warming the air by a degree or two can help dry the air out and greatly reduce the diseases on vegetable transplants.

Dr. Hausbeck’s work is funded in part by MSU‘s AgBioResearch.

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