Stem rot on garden crops can have a wide range of causes. In tomatoes and peppers, for example, it can be a symptom of several different diseases caused by fungi in the soil. In members of the cucumber family (zucchini, squash, pumpkin, gourds), it’s often the squash vine borer that causes the stems to decay.
Therefore, diagnosing and controlling stem rot depends on which garden crop is affected.
Stem Rot Caused by Fungi and Parasites
The fungi Rhizoctonia and Fusarium, and the oomycetes Pythium, a parasitic plant pathogen, live in the soil from where they attack the plant, often in the seedling stage. Symptoms include spots on the lower part of the stem, in a wide range of colors: gray, brown, black, or vibrant red. The disease leads to root decay, wilting, dieback, and weakened plants.
Another fungus that can cause stem rot is Sclerotinia sclerotiorum (White mold). It’s one of the plant pathogens with the largest host range that can befall more than 350 species. Garden crops susceptible to the fungus include beans, cabbage, carrots, cucumbers, cilantro, lettuce, melons, onions, peas, pumpkins, squash, and tomatoes.
Sclerotinia sclerotiorum can manifest itself by two distinct and opposite symptoms. One symptom can be water-soaked irregularly shaped spots on stems, leaves, and fruit. The fungus visibly spreads by covering the plant surface with a cotton-like substance, which is the mycelium of the fungus. Eventually the entire plant turns soft and slimy and dies.
The other, and very different symptom of Sclerotinia sclerotiorum are distinct but dry lesions on the stem, stalk, or branches. As the lesions grow, they girdle the affected plant part—or the entire plant, if the lesions appear at its base—and the plant turns first yellow, then brown, and eventually dies.
In either type of symptom, at an advanced stage of the disease, you might notice a small, compact mass that resembles black seeds. These are the hardened mycelium of the fungus.
In members of the cucumber family, stem rot may also be caused by another parasitic plant pathogen, Phytophthora capsici that causes Phytophthora blight. Water-soaked lesions on the vines become brown, the plant tissue dies, and the stem is girdled, which leads to wilting and dieback of the vine above the lesion.
The pathogen lives in the soil but is transmitted to plants primarily through water, such as rain or irrigation. In warm and moist weather, the fungus spreads rapidly and can kill a plant within a few days.
How to Prevent Stem Rot
As with many fungal diseases, once a plant is affected, there is not much to be done. Once you notice stem rot, there is usually already damage to the roots or other parts of the plant. That’s why good gardening practices and preventive measures cannot be over-emphasized.
Soil-inhabiting fungi like the ones that cause stem rot thrive in moisture. Poorly drained soil, often in combination with warm weather, is an ideal breeding ground.
When transplanting seedlings, plant all the plants at the same depth so that none of them is buried in the soil.
As you cultivate the soil or till, make sure that you don’t toss any soil against the stems. Tilling the soil can bring the fungus closer to the surface.
While rotating crops in your vegetable garden is always a must because it helps control other plant diseases, in the case of Phytophthora capsici and Sclerotinia sclerotiorum, it does not help because these fungi are resistant to winter temperatures and can live in the soil year after year for a more than a decade.
Always ensure proper air circulation and give plants ample pace. In rows that are too densely planted, humidity gets trapped, which propels fungal growth especially in warm weather. Thin seedlings as needed.
A trellis is a good way to keep vines and foliage off the soil and improve air circulation.
If you apply organic matter as mulch, make sure it’s full decomposed. If your compost isn’t ready yet, you are better off using plastic sheets for mulching.
Healthier plants are less susceptible to disease so make sure the plants get all the nutrients they need.
Don’t overwater, and avoid using surface water from ponds and creeks, which may be contaminated with Phytophthora capsici.
If you have a problem with any of the fungi causing stem rot, a fresh start with healthy soil in a raised bed might be the best solution.
Stem Rot Caused by the Squash Vine Borer
A sudden wilting of your squash plants is not necessarily caused by any of the fungi above. It may also be the doings of the squash vine borer, whose larvae burrow into the stem. Upon looking closely, you might see tiny holes in the stem and sawdust-like frass. If that’s the case, find out how to control squash vine borer here.