By: Bonnie L. Grant, Certified Urban Agriculturist
Bacterial diseases on plants come in many forms. Pea bacterial blight is a common complaint during cool, wet weather periods. Pea plants with bacterial blight exhibit physical symptoms such as lesions and water spots. Commercial growers don’t consider this a disease of economic importance, but in the lower-yielding home garden, your harvest can be depleted. It is best to be able to recognize the signs and symptoms and know what control measures are appropriate.
Recognizing the various diseases that can occur on vegetable plants is a challenge. Bacterial diseases come in many forms and attack many types of plants. One of the more common is bacterial blight in peas. It can spread through rain splash, wind, or mechanical methods. That means it can become epidemic in field situations. However, the symptoms are mostly cosmetic, except in very severe cases, and most plants will survive and produce pods.
Bacterial blight in peas is caused by a bacteria that harbors in the soil for up to 10 years, waiting for the right host and conditions. In addition to cool, wet weather, it is most prevalent when conditions already exist that damage the plant, like hail or heavy winds. This invites the bacteria by presenting a wound for entry.
The disease mimics several fungal diseases but cannot be managed with a fungicide. However, it is best to separate it from those pathogens. In severe infections, the pea plant will become stunted and any forming fruit will weep and ooze. Most cases will simply end when conditions dry up.
Bacterial pea blight starts out with lesions that are water-soaked and turn necrotic. The disease affects only the above-ground plant. As it progresses, water spots expand and become angular. Lesions weep initially and then dry and fall out.
It can cause tip death at certain points where the disease girdles the stem but usually doesn’t kill the whole plant. The bacteria does cause stunted growth, diminished pod production when sepals are infected and even seed infection. Once temperatures rise and rain diminishes, most cases of pea bacterial blight subside naturally.
Control starts at planting by using clean or resistant seeds. Never use seeds from infected plants. Keep all tools and machinery sanitized to prevent spreading or introducing the bacteria.
Water gently from under the leaves of the plant to prevent splashing. Do not water in the evening where leaves do not have a chance to dry. Also, avoid working in the area when it is raining or overly wet.
If you “chop and drop” old plants, wait at least two years before planting peas in that area again. Bacterial blight should be thought of like a cold and is just as contagious, but it won’t kill plants and is easy to manage with good hygiene.
This article was last updated on
Gina Foreman* and Brian Hudelson, UW-Madison Plant Pathology *Completed as partial fulfillment of the requirements for Plant Pathology 875 – Plant Disease Diagnostics Clinic Internship at the University of Wisconsin Madison.
Item number: XHT1093
What is bacterial blight? Bacterial blight, also known as blossom blight or shoot blight, is a common and often serious disease of Chinese, Japanese, Persian and common lilac, as well as walnut, apple, pear, plum and cherry. White flowering varieties of common lilac are most susceptible to the disease.
What does bacterial blight look like? Initial symptoms of bacterial blight may include dark brown necrotic (dead) leaf spots with yellow halos. If leaf spots develop before leaves are fully expanded, leaf curling and twisting may result. More advanced symptoms include necrotic blotches starting at the leaf margins and advancing inward, as well as black streaking on twigs. In its most severe form, bacterial blight can result in the death of branch tips, leaves and blossoms.
Where does bacterial blight come from? Bacterial blight is caused by the bacterium Pseudomonas syringae pv. syringae (Pss), which survives in diseased stem tissue (cankers), plant debris, and soil. Pss can be spread by insects and on pruning tools, but is more commonly spread by wind and rain. Often Pss is found on the surface of healthy plants and does not cause disease. Infections can occur when the bacterium enters tissue through natural openings, or through wounds caused by insects, pruning, wind damage or hail.
How do I save a plant with bacterial blight? Prune diseased twigs 10 to 12 inches below the point of visible symptoms, and dispose of the branches by burning or burying them. Always prune in dry weather, and after each cut, disinfest pruning shears by dipping them for at least 30 seconds in a 10% bleach solution, or alcohol (spray disinfectants that contain at least 70% alcohol can also be used).
How do I avoid problems with bacterial blight in the future? When planting lilacs, provide adequate spacing between shrubs. Thin individual shrubs each winter to promote good air circulation (see UW Garden Facts XHT1015 for pruning tips). Properly fertilize, water and mulch shrubs to avoid stress that may predispose them to disease. Avoid overhead watering that may keep leaves wet. If you have had problems with bacterial blight, you may want to use a combination of copper and mancozeb-containing fungicides for control. Apply fungicides two to three times at seven to 10 day intervals as leaves emerge, but before symptoms develop. Read and follow all label instructions of the fungicide that you select to insure that you use the fungicide in the safest and most effective manner possible.
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