Stem Blight

There are several fungal diseases that 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, in fact, that leaves stay attached to the twig, rather than falling off. This is called flagging. Generally speaking, fungicides do not control stem blight.

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


As soon as stem blight is spotted, the tissue should be removed well below the diseased area. Be sure to dip cutting tools in a 1:9 bleach and water solution, to prevent spreading the spores to healthy tissue. You can see that you’ve reached healthy tissue by looking at the cut. Infected vascular tissue will have a reddish color, 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.

Photo of stem blight in avocado from UCANR

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Fruit trees
Being shallow-rooted trees, avocados (pictured), citrus, apples and peaches are vulnerable to water stress during hot summer months. 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 a condition called “salt and pepper syndrome”. This occurs when fungal spores have entered the vascular bundle and spread to various parts of the tree, killing off random branches. Oozing cankers may also be present. This ooze is actually millions of fungal spores, so treat it accordingly. Remove infected branches as soon as they are seen and monitor closely for further damage.


Blueberries
Blueberries can be attacked by a fungus called Botryosphaeria dothidea. Young shrubs are especially vulnerable. It takes 7-10 days for symptoms to appear, and when they do, you need to work quickly to save the plant. Diseased stems will be brittle and dark brown, with dead 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 laying around and certainly do not add it to your compost pile!


Cucurbits
Cucurbits, such as pumpkin, zucchini, and melon, can be infected by the Didymella bryoniae fungus. This is called gummy stem blight. Unlike trees and shrubs, 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 in conjunction with another fungus called Phomopsis logical, which attacks seeds. Green beans, peppers and tomato plants can host these fungi without any symptoms, but planting soybeans nearby can be problematic.


Southern blight
Southern blight affects over 500 hundred plant varieties, including azalea, potato, apple, tomato, hydrangea, peanut, begonia, and marigold. 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. Our 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. Once soil is severely infected, solarization or fumigation with methyl bromide are the only known treatments. (Methyl bromide has been banned in most countries as an ozone-depleting chemical - it’s not something I would use, ever.)


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
• proper irrigation
• aerate soil
• crop rotation
• remove diseased plants
• keep mulch away from trunks and stems
• avoid mechanical damage to plants (weed-wackers, clumsy friends, heavy traffic)