It is caused by the fungus Microsphaera penicillata, which occurs on a number of different plants. Probably blueberry strains of the fungus are confined in their host range to Vaccinium species and other closely related plants, but no cross-inoculation work with the organism has been done.

The fungus grows only on the surface of living blueberry leaves. In late summer the web of fungus growth on the surface of the blueberry leaf produces tiny, dark-brown, round bodies; cleistothecia which fall to the ground with the leaves and carry the fungus over winter. Within them, in asci, the sexual spores of the fungus are produced. In spring, when the ascospores are mature, the cleistothecia rupture and release the ascospores into the air. Wind may carry them to young leaves of the blueberry. If conditions are favorable, the spores germinate and initiate small spots on the leaf surfaces. In late spring and early summer the spots remain so small that they are generally overlooked by the casual observer. The fungus continues to develop slowly, however, and when the infection is well established on susceptible varieties about the middle of the Summer, a powdery layer of summer Spores, or conidia, is produced on the leaf spots. These spores spread the fungus rapidly to uninfected areas. On susceptible varieties the fungus may cover the entire leaf surface. In late Summer the infected areas produce a new mass of cleistothecia, which repeat the cycle for the following season.

The susceptibility of different varieties and selections to the disease varies considerably. Some are badly infected,others have scarcely a single infected leaf. No variety is immune to the disease although some resistant ones do not become infected until the middle of October. Most of the named varieties that have been introduced by the Department of Agriculture since 1940 are highly resistant. Jersey, which is planted most extensively in New Jersey is very susceptible. Cabot and Pioneer, which are most susceptible to the disease, are being replaced with newer varieties that have superior horticultural qualities and resistance to powdery mildew.

The disease can be controlled by dusting or spraying. On the lowbush blueberries in Maine, applications of copper-lime dust (25 pounds of mono-hydrated copper sulfate mixed with 75 pounds of hydrated lime) at the time of blossom drop and again 10 days later will check the disease. The dust should be applied at the rate of 40 pounds to the acre to be effective.

On cultivated blueberries, cover sprays with wettable sulfur (3 pounds in 50 gallons of water) or bordeaux mixture (4 pounds copper sulfate and 4 pounds hydrated lime made to 50 gallons of spray with water) at the end of blossoming give good control. Either material should be applied at the rate of 300 gallons an acre to be effective.

STEM CANKER is injurious on cultivated and wild highbush blueberries and the wild rabbiteye blueberry in the southeastern United States. It has also been found on cultivated bushes in a few places in New Jersey.

It is caused by Physalospora corticus, a fungus that lives on the living bark of the blueberry and produces cankers on the twigs and stems. Both sexual and asexual spores are produced on the surface of the cankers over a long period during the growing season. In wet weather, spores are liberated to uninfected parts of the bush or may be blown to nearby bushes.

If weather favors infection and the spores land on susceptible blueberry tissue, a new canker will form; it appears as a reddish, conical swelling, which does not enlarge the first season. Thereafter it swells gradually. After a few years it may girdle and kill the stem. The use of infected cutting wood or rooted cuttings to establish new fields may carry the disease great distances.

Symptoms vary with the susceptibility of the infected bushes. Sometimes the fungus may penetrate the bark and make big cankers with deep cracks and fissures on the main stem of the plant. On less susceptible varieties the cankers are less extensive, they are not swollen, and they may form only on side branches.

Varieties differ in susceptibility. Only a few varieties Angola, Wolcott, Murphy, Scammell, Atlantic, Ivanhoe, Jersey, Rubel, and Rancocas are resistant enough to remain in commercial use in North Carolina.

Stem canker was first observed in North Carolina in 1938. Some fields then were already heavily infected. The disease was observed in New Jersey in 1951.

It is hard to control in North Carolina. Fungicides have been ineffective because the fungus produces spores over a long period during the season. Roguing or removal of infected bushes is of little value because the disease may become reestablished from the wild. Resistant varieties, such as Murphy, Wolcott, and Angola, offer the only possible control. In New Jersey, where the disease does not occur in the wild, the eradication of individual cankers or entire infected bushes probably would eliminate the disease.

SEVERAL OTHER DISEASES have been reported on blueberries in North America and Europe. Most are local or serious mainly on Vaccinium species other than the cultivated highbush blueberry.

Witches'-broom, or stem rust, is caused by the fungus Pucciniastrum goeppertianum, a heteroecious rust with fir, Abies species, as the alternate host.

It occurs on cultivated and wild blueberries in New England and on wild blueberries in many places. It produces several short, swollen twigs on infected bushes, which are crowded on a small area near the point of infection. Heavily infected bushes produce no fruit. Possibly the disease can be controlled by removing all the true fir trees from the vicinity of the blueberry field. Pruning the infections from bushes might also be of some value.

Leaf rust is caused by the fungus Pucciniastrum myrtilli, another heteroecious rust with hemlock, Tsuga, as the alternate host. It normally occurs on various blueberry species only in areas where hemlock trees are present. In some seasons it occurs in severe outbreaks in cultivated fields far removed from the alternate host. The fungus can evidently be spread rapidly by uredospores after an infection is once established. The fungus produces small, irregular, dark-brown spots on the leaves. The spots may become numerous later in the season and cause early defoliation. No control is known for the disease on cultivated highbush blueberries, but copper-lime dusts applied at the time of blossom drop, followed by an additional application to days later, have been effective in controlling the disease on lowbush blueberries in Maine. This is the same treatment that is used against powdery mildew.

Several leaf-spot diseases have been reported on blueberries in North Carolina and other parts of the Southeast. The most important is double spot, caused by the fungus Dothichiza caroliniana. The disease occasionally causes severe defoliation in North Carolina and must be controlled by spraying with bordeaux mixture.

Crown gall of blueberries is caused by the bacterium Agrobacterium tumefaciens, which produces galls on many other plants. It is found on highbush blueberries in New York, New Jersey, Michigan, Washington, and British Columbia. It occurs on plants in the field, in nurseries, and in cutting beds.

It produces swollen galls along the stems and small twigs and occasionally at the base of the canes near the ground. The galls are irregular and of various sizes. Often the gall involves the entire circumference of the branch and affected branches and twigs die. In Washington certain varieties and selections are badly infected, while plants of a different selection in adjacent rows may be healthy. Although various other host plants become infected when inoculated by bacteria isolated from blueberry galls, the blueberry strain of the crown gall organism appears to be best adapted to the blueberry. The disease is probably spread in the field or nursery when plants are pruned or when softwood cuttings are made. No attempts have been made to control the disease in blueberry fields.

Tip blights of blueberries occasionally are common. They are probably caused by winter injury, excessive soil moisture, or other environmental factors that weaken the bush. Frequently stem tips of bushes, weakened by other causes, are infected by weakly parasitic fungi. Diaporthe (Phomopsis) vaccinii is reported to be a cause of tip blight in North Carolina, New Jersey, and New England. That fungus is not often encountered, however, and is only one of a number of fungi that may be found in blighted tips. In Washington Diaporthe vaccinii Occurs as a saprophyte on dead blueberry tips.

AUSTIN C. GOHEEN, a native of Washington State, received his undergraduate training at the Western Washington College of Education and the University of Washington and his graduate training at the State College of Washington. He has been with the Bureau of Plant Industry, Soils, and Agricultural Engineering since 1950. He has worked on blueberry and cranberry diseases in New Jersey in cooperation on with the New Jersey Agricultural Experiment Station. He was transferred to the Plant Industry Station at Beltsville in 1953.