Monday, February 11, 2008


The Rev. Dana Prom Smith, S.T.D. Ph.D. (2/11/08)

Murphy’s Law, named after the aerospace engineer Edward A. Murphy, Jr., reads “If there’s more than one way to do a job, and one of those ways will result in disaster, then somebody will do it that way.” In other words, someone will always let a slice of bread fall to the floor, buttered side down. While originating in aerospace engineering, Murphy’s Law is applicable to gardening, especially to growing tomatoes and that an annoying plague on tomato plants, early blight with its circular lesions, cankers, and fruit rot.

Murphy developed something else equally as important, called “defensive design,” a way of avoiding disasters that aren’t necessarily inevitable.

Defensive design in gardening begins with sanitation. As in the bathroom and kitchen, the first line of defense against disease is picking things up and washing things off. The spores of the fungus hang out unseen in plant debris, even over the winter, so it’s always pick-up time in the garden.

They also stay in the soil of a diseased plant. Another aspect of Murphy’s defensive design is rotating crops, a practice beneficial for many reasons. Tomatoes should be planted in different beds each year for at least three years. If containers are used, clean the container thoroughly and change the soil every year. Soil can be sterilized with hot water or by covering the soil with black plastic sheets or black plastic bags over the containers for a couple of weeks. However, it’s safest to change soils.

The next defensive design is air circulation. Space the tomatoes far enough apart so that air circulates within the vine. Non-productive branches, branches without blossoms, can be pruned to increase air circulation in the plant. A word of caution: too much pruning will expose the fruit to sunburn.

Overheard watering, such as sprinkling, will likely foster early blight spores and burnt leaves. Dusty leaves are allowable in defensive design sanitation.

Early blight comes about because of conditions favoring the fungus Alternaria solani. In Flagstaff, those conditions are the monsoon season, July through September, when the winds circulate from the south and southeast rather than the west and northwest, bringing moisture from the Gulf of Mexico and the Gulf of California. In other words, Alternaria solani like it moist and hot. Sadly, blight strikes only after gardeners have watched a tomato plant come to maturity. Grief happens.

Tomatoes are best planted early during the dry season so that they have time to develop and thus can more likely resist infestation. If beginning with seeds, make sure that the seeds are certified as disease-free. If seedlings are purchased from a commercial nursery, choose disease resistant varieties and inspect them with a fine-tooth comb. Commercial nurseries like hospitals are hotbeds of disease.

If old tomato plants are used in compost, make sure that they aren’t blighted. It may be wise not to use that compost to enrich the soil where tomato plants will be grown again, just in case spores were undetected.

Since the Alternaria solani are invisible to the naked eye, early blight can only be detected with magnification or after it’s begun. The signs are brownish black bulls-eye circular lesions on the older leaves, and as the blight grows the tissue around the spots may turn yellow. If the affliction is advanced, the stems will be girdled with the cankers and the fruit will rot with freckled and spotted lesions. This means daily inspections.

If these defensive designs are breached, then treatment is possible, such as sulfur dust or the fungicides chlorothalonil, commercially named Bravo and Daconil, and azoxystrobin, known as Quadris. As with any pesticides, read the labels and follow the directions exactly.

Sometimes, defensive designs don’t work and the slice of bread hits the floor buttered side down. At the first sign of infestation, cut, prune, and send the blighted leaves, stems, and fruit to Environmental Services, remembering the times when ripe dusty tomatoes were plucked warm from the vine on sunny afternoons and eaten, bent over with chin-dripping juice.

Copyright © Dana Prom Smith

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