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Posted 12 July 2010. PMN Crop News.

Expect Disease Problems on Potatoes and Tomatoes

Source: North Dakota State University Press Release.

Fargo, North Dakota (July 6, 2010)--There needs to be a susceptible host, the presence of the pathogen in the immediate environment and sustained conditions favorable for the pathogen to develop


The cool and wet conditions our region has been going through may be doing a good job of keeping the grass green and the water bill in check, but be prepared for a possible outbreak of potato-tomato late blight fungus.

Late blight has been observed on tomatoes in home gardens in Michigan and Manitoba and in commercial potato fields in southeastern North Dakota, as well as western and central Manitoba.

“Keep in mind that for any disease to develop, three factors need to exist,” says Ron Smith, North Dakota State University Extension Service horticulturist. “There needs to be a susceptible host, the presence of the pathogen in the immediate environment and sustained conditions favorable for the pathogen to develop.”

Even though the term late blight implies arrival later in the season, the beginning symptoms can be traced back to early July if the right conditions are present. Generally, the symptoms are seen on the older foliage, but it can develop on any part of the tomato plant. The most obvious symptom is a powdery white growth that shows up on the bottom of the leaves and contains the spores that can be spread by wind, water splash or human activity.

Potato tubers with the problem turn a reddish brown and become dry and granular. Secondary pathogens, such as bacteria, then move in and quickly cause the tuber to turn soft and rot.

Home gardeners can help control this disease using the following approaches:

• Use excellent sanitation practices such as keeping weeds out of the garden and cleaning up the previous year’s crop residue.

• Plan a good crop rotation. Growing potatoes followed by another member of the nightshade family, such as eggplant, peppers or tomatoes, is not a valid cycle. Move out of the potato family for a period of three years. For example, plant potatoes followed by peas, beans and cabbage and then go back to potatoes. This will help break the disease cycle.

• Avoid water splash when irrigating the garden. Drip systems can be installed easily and also conserve water. If overhead watering must be used, do it in the morning hours to give the foliage a chance to dry.

• If a history of disease problems exists with tomato or potato plantings in the garden, use fungicides that contain the active ingredient chlorothalonil. It is a preventative fungicide, so it must be applied before the disease appears. It cannot cure any visible symptoms that have begun to show up. Chlorothalonil is not a systemic fungicide, so complete coverage of the plants is necessary to be effective.

• If a plant becomes severely infected, remove it completely and carefully. Place a plastic garbage bag over the infected plant and pull the plant out of the ground. Tie the bag immediately to keep the spores from spreading.

• When selecting plants for the garden, try to find those that are noted for their resistance to this pathogen. Tomatoes are bred to be resistant to many of the common diseases, such as verticillium and fusarium wilts, but not to late blight fungus.

“North Dakota is a primary potato-producing state in the country,” says Nick David, NDSU Extension potato pathologist. “Late blight in the garden can escape to commercial fields nearby and cause severe economic losses. Remember that this pathogen caused thousands of deaths during the Irish potato famine of 1845 and 1846.

Nick David