Blueberry bushes are popular with gardeners for their attractive bushes and delicious, healthful fruit. A low-maintenance plant, blueberries are sometimes vulnerable to certain conditions and diseases that can cause them to exhibit spotted, flecked or blotched leaves. Before you decide on a course of treatment--whether it be correcting poor soil conditions, treating a fungal disease or destroying some plants to save the rest--you need to find out what is causing the spots. With some basic observational techniques, you can be well on your way to a correct diagnosis and treatment of the cause of your blueberry bushes' spotted leaves.
Check the size of the spots on the leaves of your blueberry bushes. The fungal disease anthracnose begins as small reddish flecks on the young leaves and tender shoots. In later stages of the disease, the dark red or maroon lesions measure more than 1 inch across. These lesions eventually turn dark brown--as does the stem--and there is severe dieback. If the spots are quite small--1/8 to 1/2 of an inch across, with tan centres and purple borders--it is more likely that your blueberry bushes have Septoria leaf spot, which also causes poor growth, leaf loss and small reddish spots.
Treat both anthracnose and Septoria leaf spot with fungicide sprays. According to the North Carolina State University Extension, either captan or benomyl can control leaf spot. Treat the plants once before the harvest, then repeat sprayings every 2 weeks from harvest until mid-August.
Check whether the spots are on the bottoms of the leaves as well as the tops to distinguish between powdery mildew and red ringspot; these two diseases have almost identical symptoms. Powdery mildew, which causes leaves to first pucker and then display small, yellow, reddish-bordered spots, will affect both the tops and bottoms of leaves. Red ringspot, which also features reddish-bordered circular spots, usually only appears on the tops of the leaves. If you live in New Jersey, a diagnosis of red ringspot is more likely.
Consider the time of season to further distinguish between powdery mildew and red ringspot. Powdery mildew usually begins in July; red ringspot appears in August and September.
Treat your blueberry bushes with fungicides if they have powdery mildew. For red ringspot, the only option is to pull up and destroy the affected bushes.
Check the undersides of any red-spotted leaves for a whitish growth of fungus, which would indicate red leaf disease. This incurable blueberry bush disease starts as a red spot or blotch, which spreads to the entire leaf. After turning red, affected leaves blacken and die.
Pull out and destroy any blueberry bushes affected by red leaf disease to spare the rest of your crop.
Check the shape of the leaves for clues as to the source of the spotting. The disease known as "shoestring" sometimes causes a deformity that makes leaves thin and long, hence the name. Crescent-shaped deformities are also common. The leaves will feature shiny, purple spots, specks, and blotches, with long reddish streaks on the stems of the plants.
Remove and destroy any infected bushes if your blueberry plants have "shoestring," and use an aphicide to treat the rest of them to kill the blueberry aphid that carries this disease. Repeat the spraying at 2 or 3 week intervals.
Evaluate your use of nitrogen fertiliser. Blueberry plants with leaves that develop red spots after first turning yellow may be suffering from a lack of nitrogen.
Consult your county extension office or a reputable plant nursery for further guidance if you are unable to identify the cause of spotting on your blueberry bushes.
When using fungicides or pesticides, read all labels carefully, and follow directions exactly. Do not use Neem oil to treat fungal infections of your blueberry bushes. It can damage the protective wax on the leaves, making your plants even more susceptible to disease.