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In my last article I wrote on weather affecting the quality and production of garden produce. I was mainly writing about the hot and dry weather most of us were experiencing. Tomatoes were showing signs of catface and blossom-end rot. These conditions are due to uneven heat and moisture.

Since that article most of us have received more than four inches of rain followed by warm and high humidity. This weather is prime for tomato fungal diseases like early blight, septoria leaf spot, and anthracnose. Michelle Grabowski, Regional Extension Educator, has recently written an article on these diseases.

Early blight is caused by a fungus that attacks leaves, fruit, and stems of tomato plants. This disease typically starts in the lower leaves as dark brown or black spots with dark rings in the center that makes it look like a target. These spots start small but can grow to be 1/2 inch in diameter. Spots can grow together to cover the leaf causing it to dry up and fall off the plant. Stem infection and fruit infections are also black often with dark growth rings inside. Fruit spots typically start where the stem connects to the fruit and can grow to cover the top half of the fruit. Early blight has been shown to be more severe when the plant has been stressed by drought. Therefore, it is important that tomatoes are watered, especially during a dry spell.

Septoria leaf spot is a fungus that causes only leaf spots. The leaf spots are small, 1/16 to 1/8th of an inch in diameter. They have a dark edge and tan centers. Small black dots can often be seen in the center of the leaf spot. Leaves with many spots fall off the plant. In the right weather conditions, a tomato can lose the majority of its leaves from this disease, resulting in poor quality or sunburned tomatoes.

Anthracnose is a fungus that primarily causes spots on ripening fruit. Fruit spots from anthracnose are sunken water soaked spots often with a dark black center that will ooze pink spores in wet weather. This fungus rots the fruit from the inside and often makes it inedible.

All three of these diseases have several things in common so that when you are controlling one you are often controlling the other two as well.

First, all three fungi survive in the soil and on plant debris from last year’s tomatoes and weeds.

Second, they all have spores splashed up into the lower leaves to start the disease.

Third, they all need moisture to cause the disease.

Knowing these three things, there are several cultural practices that can significantly reduce the amount of disease from these three fungi.

Plant tomatoes where no tomatoes, potatoes, peppers or eggplants have been for the past three to four years.

Completely remove old tomato plants from the garden at the end of the season or bury old vines so soil microorganisms can break down the plant material.

Keep tomato leaves as dry as possible.

Mulch tomato plants with 3-4 inches of organic mulch like leaves, hay or woodchips. This keeps water in the soil where roots can get it. Mulch also reduces humidity in the plant canopy.

Water early in the morning so the leaves dry quickly in the sun and water at the base of the plant. If possible, stake or cage plants. Moisture is highest at the soil surface; supported tomato plants dry quicker and are less likely to have disease problems.

Space plants for enough away from each other so the air can flow between them. Mulching also helps so the spores don’t splash from plant to plant.

Prune heavily infested leaves off tomato plants and throw them in the garbage. Never remove more than one-third of a plant’s leaves.

If you feel you need to use a fungicide, do not apply until the disease is actually present. Organic options include copper and Serenade Garden Defense. Serenade is a biological control bacteria that protects leaves from fungi. It is new to the market but is labeled for tomato early blight. Sulfur is another organic fungicide but can burn leaves if applied at high temperatures.

Conventional options include products with Mancozeb or Chlorothalonil listed as the active ingredients. Whether organic or conventional, always read the label and follow the instructions before using any chemical control products, paying close attention to how long you need to wait to harvest after application.

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It’s the time of year when County Extension Offices around the state are taking applications for the Master Gardener training program to be held in early 2007. Chisago County residents interested in becoming Master Gardeners can get more information and an application by attending our monthly meeting at 6:30 p.m. either Tuesday, Aug. 22 or Sept. 26, at the Senior Center in North Branch. You can call the office at 651-674-4417 for directions.

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