Fielding questions: How can I avoid tomato diseases next year?

Gardening columnist Don Kinzler answers questions about preventing tomato disease, the dangers of using black walnut leaves in mulch, and more.

tomato disease October 29, 2022.jpg
This week a reader asks garden columnist Don Kinzler for tips to avoid tomato disease.
Contributed / Don Kinzler
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Q: How can I avoid tomato diseases next year? The photo shows what happened. – Lucia S.

A: Tomato diseases can affect both leaves and fruit, and the most common diseases are Septoria leaf spot and Alternaria early blight. Prevention is the key to control, because once tomato plants are affected, the disease can’t be cured, only mitigated somewhat.

Tomato disease prevention begins this fall. Because disease organisms can survive winter on old stems and leaves, remove tomato plants from the garden and dispose off site. Next spring, rotate tomato plants to a different spot within the garden, if possible.

Many tomato cultivars have disease resistance, which is a good first line of defense, including Celebrity, Big Beef, Better Boy, Early Girl, Fantastic, Mountain Series, Park’s Whopper and others. Disease resistance doesn’t mean total immunity, though, so additional control measures are wise.

Avoid overhead watering, which can splash disease organisms from soil onto leaves. Instead, water only the soil. Soaker hoses are ideal. Water early enough in the day so foliage and stems dry before nightfall.


Staking and pruning will increase air flow and help keep vines dry and be less disease susceptible. Lower leaves that contact the soil can be removed. Diseases usually start on the lower leaves and progress upward.

Mulch around plants with straw, landscape fabric, or grass clippings that are herbicide-free. Mulches will conserve moisture and reduce diseases and blossom end rot disorder.

If you are consistently troubled with disease, vegetable fungicides can be applied while the foliage is still healthy. Fungicides can prevent disease organisms from establishing or progressing, but can’t reverse damage already done. Look for products containing chlorothalonil or copper.

Q: I usually shred leaves and put them on my garden. Will black walnut leaves impact or restrict growth of plants in my garden the rest of the year? I know walnut trees give off some kind of chemical than impacts certain plants under the tree. Will the leaves hurt my garden? – Gary H.

A: Leaves are a great addition to garden and flowerbed soil in the fall. Black walnut leaves, though, are among the types of leaves that should be avoided.

Black walnut trees contain the compound juglone, that is produced in the branches, leaves, fruit and root system. Juglone is well-documented to cause injury or death to susceptible plants growing in the vicinity and exposed to the black walnut’s juglone.

Some university sources indicate that juglone in leaves will break down within several months when composted; other university sources aren’t so sure. Much probably depends on the conditions, moisture and temperature at which the leaves are decomposing.

Because it’s unknown definitively when or if juglone in leaves will effectively break down, the safest route is to keep black walnut leaves out of garden or flowerbed soil. It would be a shame to introduce a compound into the soil that would be toxic to the vegetables or flowers we treasure.


Instead, skip the black walnut leaves, and incorporate leaves from other tree species into the soil. Leaves decompose rapidly in the soil by spring, adding a good source of organic material.

Q: What’s the best way to store canna roots over the winter. We have a heated garage, but it will drop below 20 degrees F. at times. – Nicole W.

A: The University of Minnesota provides the following canna storage information, which I’ve found successful: After digging the rhizomes, cut stems back to two inches and let them dry. Leave in a box or cool part of the house where they won’t freeze, such as a basement where the temperatures range between 40- and 50-degrees F.

Every few years the rhizomes can be divided. When dividing, each piece must have an eye, or growing point, on it.

More gardening columns from Don Kinzler

If you have a gardening or lawn care question, email Don Kinzler, NDSU Extension-Cass County, at . Questions with broad appeal may be published, so please include your name, city and state for appropriate advice.

Don Kinzler, a lifelong gardener, is the horticulturist with North Dakota State University Extension for Cass County. Readers can reach him at
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