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Ask the Master Gardener: Warm, humid conditions can lead to septoria leaf spot on tomato plants

In combatting septoria leaf spot, it's very important to keep tomato leaves as dry as possible.

Tomato plant leaves with septoria leaf spot.
This tomato plant is infected with septoria leaf spot, one of the early diseases which affect tomato plants. (Doug Oster/Pittsburgh Post-Gazette/TNS)
Doug Oster/TNS
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Dear Master Gardener: I’ve been fighting a disease on my tomato plants all season. I keep cutting off diseased leaves until there are only a few remaining on the plant. The leaves are yellow with spots on them. I have two cherry tomato plants that hadn’t been affected but now they’re also getting diseased. I sprayed a fungicide, but it doesn’t seem to be helping. What can I do?

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Answer: It sounds like the problem with your tomatoes could be septoria leaf spot, which is a common tomato disease found in Minnesota. The Brainerd lakes area has had warm, wet, humid conditions, which is conducive for septoria leaf spot. Most tomato leaf spot diseases overwinter in the soil and then splash onto the lower leaves of the plant, so symptoms usually appear on the lower leaves first. Then, the disease spreads upwards from the oldest to the newest growth. Typically, there are many spots per leaf, then leaves turn yellow, then brown, and then wither. Tomato plants can tolerate high levels of leaf loss from leaf spot diseases without affecting the tomatoes.

Before using a fungicide, it is imperative that the problem be diagnosed correctly. Is the problem just due to cultural practices or weather? Is it a virus, bacteria, or fungus? Which virus, bacteria, or fungus is it? What is causing it? Most home garden tomatoes do not need to be treated with a fungicide.

It's very important to keep tomato leaves as dry as possible. Pinch off infected leaves to keep leaf spot diseases in check. It’s also important to rotate crops — plant tomatoes where no tomatoes, potatoes, peppers, or eggplants have been for the past 3-4 years. Do not save seed from infected plants.

Dear Master Gardener: I started seeds in small seed starting pots with partially finished compost. I also used partially finished compost when I direct-seeded in my garden. I’ve had poor results. Is it bad seed or bad compost?

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Answer: I’m betting on compost as the problem. First let’s address starting seeds in pots. The U of M recommends starting seeds in a commercial seed-starting mix, which is usually composed of vermiculite and peat, without any true soil. These mixes are sterile, lightweight, and free from weed seeds, with a texture and porosity especially formulated for germinating seeds and tiny seedlings. Partially decomposed compost doesn’t meet any of those requirements — it’s not sterile, lightweight or free of weed seeds. Composting is a process that allows naturally occurring microbes to convert yard waste such as leaves, grass clippings and plant material to a useful organic soil amendment or mulch. Incorporating compost into light, sandy soil (which is often found in this area) helps it hold moisture and nutrients. Incorporating it into heavy soil improves drainage. Finished compost is dark brown or black, crumbly, has an earthy smell, and the organic items that have been added to the compost pile are no longer recognizable. Unfinished compost is a mixture of organic materials that are still recognizable and drawing nitrogen to finish the breakdown process — potentially starving your seeds. Unfinished compost should only be used as a top-dressing/mulch during the growing season.

Dear Master Gardener: My sister and I live in different parts of Minnesota. She sees lots of grasshoppers in her gardens and I don’t. Are they good or bad insects? If they are bad, should pesticides be used to get rid of them?

Answer: Grasshoppers are one of the most familiar insects. There are over 550 species of grasshoppers in North America, but only a few damage gardens. There are four in the genus Melanoplus that are particularly harmful: Twostriped Grasshopper, Differential Grasshopper, Migratory Grasshopper and Redlegged Grasshopper. All four can be found in Minnesota. The Redlegged Grasshopper is the only one of the four that can be found in Crow Wing County but is quite abundant in southeastern Minnesota. The grasshopper population changes with rainy weather. You usually don’t see large grasshopper populations in areas that have received lots of rain because wet weather promotes a fungus that kills grasshoppers.

If grasshoppers do any damage in the garden, it is often insignificant. Grasses are particularly favored, but most garden plants can be damaged. The plants most commonly harmed by grasshoppers are beans, leafy vegetables, corn and iris. Most feeding occurs on foliage. Grasshoppers are usually not worth treating over a few holey leaves. They’re also large and very mobile, so using pesticides on them is usually ineffective. Most importantly, pesticides can have unintended effects on pollinators and other beneficial insects.

Dear Master Gardener: I planted some herbs this year but my basil is declining. Does basil get a particular disease?

Answer: First, check to see if the problem is aphids, as this is a common problem for basil plants. If your basil plants started out looking healthy, then at 6-12 inches the growth became stunted, you saw brown streaks on the stem, brown blotches on the leaves and the plant wilted, then it is most likely a disease called Fusarium wilt. Fusarium wilt is a fungal disease caused by a soil-borne pathogenic fungus that is found in basil only. Eventually the plant wilts and curls over and the infected leaves fall off.

There is no treatment for infected basil plants. Infected plants should be removed at once to prevent other nearby basil plants from contracting the disease. Prevention is the key to having healthy basil plants. Water early in the day and keep the foliage as dry as possible. If you plant your herbs in pots, make sure to use new soil each year and sterilize the pots. If you plant herbs in your garden, then rotate the herbs to different places and remove plant debris in the fall. The best plan of action is to buy a variety that is tolerant or resistant to Fusarium wilt. The varieties most susceptible to the disease are sweet basil and Spicy Globe. Some of the specialty basils like lemon basil and purple basil are more disease-tolerant. A variety of sweet basil that is resistant to Fusarium wilt is Nufar, which is available through several seed companies.

You may get your garden questions answered by calling the new Master Gardener Help Line at 218-824-1068 and leaving a message. A Master Gardener will return your call. Or, emailing me at umnmastergardener@gmail.com and I will answer you in the column if space allows.

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University of Minnesota Extension Master Gardeners are trained and certified volunteers for the University of Minnesota Extension. Information given in this column is based on university research.

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