Stem Blights

Several fungal diseases result in stem blight. Once they enter, usually through a wound, fungal spores spread through the xylem and phloem (plant veins), clogging the arteries. Without free-flowing sap, twigs quickly die of dehydration and starvation. They die so fast that leaves stay attached to the twig rather than falling off in a behavior known as flagging. Generally speaking, fungicides do not control stem blight.

These spores are already everywhere. Plants that are water-stressed or injured become susceptible to infection. Keeping plants healthy in the first place is the best prevention.

Once stem blight is spotted, remove the infected tissue well below the diseased area. Be sure to sanitize cutting tools with a household cleaner to prevent spreading spores to healthy tissue. You can see that you’ve reached healthy tissue by looking at the cut. Infected vascular tissue will be reddish, while healthy tissue will be white or green. Monitor plants closely for signs of further die-off.

Since there are so many types of stem blight, we will look at the plants most commonly affected:

Fruit trees

Being shallow-rooted trees, avocados, citrus, apples, and peaches are vulnerable to water stress during hot summers. Proper irrigation can help them to protect themselves against Botryosphaeria and Neofusicoccum parvum fungal infestations. Initial symptoms include reddening and wilting of leaves. Once infected, trees may display salt and pepper syndrome. It is fungal spores that have entered the vascular tissue and spread to various parts of the tree, killing off random branches. Oozing cankers may also be present. This ooze is millions of fungal spores, so treat it accordingly. Remove infected branches and monitor closely for further damage.

Blueberry

The Botryosphaeria dothidea fungus can infect blueberry bushes. Young shrubs are especially vulnerable. But symptoms do not appear until seven to ten days after infection. When they do, you need to work quickly to save the plant. Diseased stems will be brittle and dark brown, with leaves still attached. Cut 8-12” below the affected area and treat the trimmings like toxic waste. Bag it or burn it, but don’t leave it lying around. And keep it out of your compost pile.

Cucurbits

Cucurbits, such as pumpkin, zucchini, and melon, can be infected by the Didymella bryoniae fungus in a disease known as gummy stem blight. Unlike trees and shrubs, most cucurbits show symptoms on leaves, stems, and fruits. Round, brown lesions and cankers exude a brown ooze that can contain millions of fungal spores. Gummy stem blight can affect plants at any stage of development. Fungicides can be used as a preventative measure but are only marginally effective.

Soybeans

Pod and stem blight in soybeans is caused by Diaporthe phaseolorum sojae. These fungi work alongside another fungus called Phomopsis logical, which attacks seeds. Beans, peppers, and tomato plants can host these fungi without symptoms, but planting soybeans nearby can be problematic.

Southern blight

Southern blight affects over 500 plant varieties, including apples, marigolds, peanuts, potatoes, and tomatoes. Also known as southern stem blight or southern stem rot, Sclerotium rolfsii kills more plants in the south than any other pathogen, along with root knot nematodes. 

Warm weather allows this fungus to attack plants at or below the soil line. Also known as white mold, the spores produce white mycelia that can be seen around infected plants and in and on the soil. Soil solarization or fumigation with methyl bromide are the only known treatments for severe infections. (Most countries banned methyl bromide as an ozone-depleting chemical, and it’s not something I would ever recommend.)

Rather than risking the use of chemicals or the loss of desirable plants, these good cultural practices can help prevent stem blight:

• Plant only certified clean seeds and plants
• Practice proper irrigation.
• Aerate the soil as needed.
• Employ crop rotation.
• Remove diseased plants.
• Keep mulch away from trunks and stems.
• Avoid mechanical damage (weedwackers, clumsy friends, heavy traffic).