Blight is the common term for any number of plant diseases typified by a spotted lesions on various parts of the plant, often in conjunction with wilting foliage and other deformities. Blights may be caused by fungal or bacterial pathogens and are not easily treated.
Bacterial Blights
Caused by a soil-dwelling bacteria, this type of blight affects a great number of species, especially woody trees and shrubs. The bacteria invades the plant's tissue, often causing open lesions, and prevents the plant from transporting water and nutrients through its vascular system, eventually leading to death.
Identification
The visible symptoms of bacterial blight varies tremendously among species.
- Oozing cankers are a common manifestation of bacterial blight on many fruit trees and other woody plants - these appear as open sores along the branches that bleed a gummy sap.
- Dark-colored lesions on flowers, fruits, and stems are common in other cases.
- Dark, irregularly shaped spots on the leaves, followed by wilting, is one common sign. This type of bacterial blight is often called leaf blight or leaf spot, names which are also used in reference to other blights caused by different organisms.
Treatment and Management
There are no natural or chemical sprays that effectively treat bacterial blight, meaning the only course of action is to manage the disease to minimize harm.
Manage Water Exposure
In general, bacterial blight is more common in wet, humid environments. It is spread to the plants when water, from rain or irrigation, hits the ground and splashes muddy droplets onto the plant containing the bacteria. Thus mulching the ground to prevent the splashing effect and avoiding overhead irrigation are two of the most helpful tricks to avoiding an outbreak of bacterial blight.
Cut Off Infected Portions
Once symptoms have been noted, it's important to cut off the infected portions of the plant and dispose of them immediately to prevent further spread. Once the disease has spread throughout the majority of the plant, it is unlikely to survive and should be removed so as not to spread the disease to other plants nearby.
Fungal Blights of Tomatoes
There are several devastating fungal blights that attack tomatoes, some of which attack other species as well.
Early Blight
Early blight appears as pea- or dime-sized brown spots on the leaves that are lighter toward the center of the spot but exhibit a distinctive yellow border around the outer edge of the spot. The stems and fruits will develop dark, mushy spots as the disease progresses. Early blight typically starts to affect tomatoes in early summer.
Late Blight
Late blight shows up later in the summer and is apparent by brown edges on the leaves of the plants, rather than spots in the center of the leaf. It can progress to engulf entire leaves, stems, and fruits, with the infected areas becoming covered in mold as they die. Late blight affects other species in the tomato family as well, such as potatoes and petunias.
Septoria Blight
Septoria blight, also known as septoria leaf spot, causes small dark spots less than an 1/8-inch in diameter all over the leaf. Unlike the other two tomato blights, it doesn't affect the fruits or stems.
Treatment and Management
Various cultural practices limit the spread of tomato blights and chemical treatments can also be employed to salvage a crop.
- Mulching around tomato plants prevents the fungal spores from splashing up onto the leaves.
- Moisture is the principal vector for the disease -- irrigate at ground level to keep the leaves dry.
- These blights generally start near the ground and work their way up the plant as the season progresses so remove the lower leaves as they become infected to slow the spread upwards.
- Fungicide sprays applied on a one to two week schedule are unlikely to completely eradicate these blights, but they can significantly reduce the damage.
- If the blight is severe in one year, avoid planting tomatoes in that location for at least two or three years to allow the build up of the pathogen in the soil to diminish.
- Clean up all the crop debris left by the tomato plants at the end of the season and dispose of them.
Corn Leaf Blight
This fungal blight causes thin tan or brown streaks in the leaves of young corn plants and can weaken the plant but does not usually kill it. It is mostly a concern in agricultural settings though it can show up in home gardens.
Treatment and Management
The best way to avoid corn leaf blight is to plant resistant varieties and look for seed that is certified disease free. The blight overwinters in the crop residue, so be sure to dispose of decaying cornstalks and leaves at the end of the season. If the infestation is severe, consider planting corn in a different part of the garden for several years.
Fire Blight
This bacterial disease is extremely common and can affect any member of the rose family, which includes apples, pear, peaches, strawberries, and many ornamental plants. It is most frequently a problem with apple and pear trees, however. In severe cases the affected plants may die.
Symptoms
The most telling symptom of fire blight is the sudden appearance of black or brown leaves and twigs at the tips of the branches -- they literally look as though they were singed by a fire. Other symptoms include twigs that appear deformed, growing with a downward hook-like pattern rather than splayed straight out. Lesions may also appear on the fruit, flowers and bark.
Treatment and Management
There are several ways to avoid and minimize the damage caused by fire blight.
- Plant resistant varieties - Of the plants that are susceptible to fire blight, none are completely resistant, but there are varying degrees of susceptibility.
- Remove infected wood - Cut off the diseased branches eight or 10 inches below the visibly infected area and dispose of them.
- Practice good hygiene - Clean up fallen leaves, twigs, and fruit at the end of the year and sterilize pruning equipment between each cut when working on infected trees. A 10 percent bleach solution works for cleaning purposes.
- Apply copper sprays (also known as Bordeaux mixture) while the trees are dormant to limit the yearly outbreak of fire blight.
Living With Garden Blights
Blights are fact of life for gardeners. Prevention and management are the keys to avoiding severe damage when there is an outbreak though minor degrees of blight can usually be tolerated.
