Leaf spot diseases cause considerable damage each year to blueberries in North Carolina. Most damage is caused by anthracnose and septoria leaf spots. Variation occurs between cultivars and between growth flushes, with the most infection occurring on the earliest flush of growth, and little or none on the last flush of growth in late summer.
Symptoms and Disease CyclesGloeosporium Leaf Spot or Anthracnose (Gloeosporium minus) Most commercial blueberry plantings in eastern North Carolina are affected with Gloeosporium leaf spot which can cause severe defoliation and reduction in yield. This disease can produce flecking, leaf anthracnose and stem lesions. Gloeosporium leaf spot was first described as a stem and leaf fleck disease of blueberry. Symptoms first appear as small reddish flecks on young leaves and stems of succulent shoots. Leaf and stem flecks do not develop further. With the leaf anthracnose stage, large (1/2 to over 1 inch across) brown lesions are the characteristic symptoms in the field. Development of lesions results primarily from infection through hydathodes at the margins of the leaf, and to a lesser extent through wounds. Secondary stem lesions can develop from leaf infections by growing through the petiole into the stem, and from bud infections late in the season. Stem lesions first appear on current season's growth as dark red circular to elliptical lesions around leaf scars. As the lesions enlarge, the affected stems turn brown and eventually become gray and die. On highly susceptible cultivars such as Jersey, the disease results in severe dieback, measuring up to 20 inches.
Septoria Leaf Spot (Septoria albopunctata) is prevalent on most of the cultivated highbush and rabbiteye blueberry cultivars grown in North Carolina. Damage is also serious in rooting beds, causing severe defoliation and poor growth. The fungus overwinters in infected leaves on the ground and in stem lesions. Spores are produced in abundance on new lesions throughout the season with heaviest infection occurring between June and September. Spots on leaves are usually small (1/8 to 1/2 inches across) with a white to tan center and a purple border. Stem lesions are flat or slightly sunken with a tan to gray center and a reddish brown margin. Stem lesions and leaf spots are most severe on small plants or on the lower parts of large plants.
Double Spot (Dothichiza caroliniana) is initially 1/16 to 1/8 inches in diameter. In late summer, the fungus develops a secondary necrotic area around the original lesion spreading in an irregular to fan-shaped pattern. The dark border surrounding the original lesion persists and the large necrotic area surrounds or is to one side of the original lesion.
Gloeocercospora Leaf Spot (Gloeocercospora inconspicua) is of minor importance as a leafspot disease in NC. It appears on the foliage by mid-summer as large dark brown, circular to angular lesions surrounded by a dark border.
Alternaria Leaf Spot (Alternaria tenuissima) occurs primarily in the spring during prolonged periods of cool wet weather, when spores are produced in abundance. In most cases only lower leaves are affected; however, instances do occur when severe infection completely defoliates the plant. Because of its importance as a postharvest fruit decay organism, an increase of the fungus on blueberry leaves in May can result in considerable damage to fruit after harvest. Leaf symptoms appear as circular to irregular-shaped brown to gray lesions surrounded by a red border. Prolonged periods of high humidity favor disease development.
Powdery Mildew (Microsphaera alni) usually does not develop on blueberry leaves until midsummer after the crop is harvested. The disease may appear as a white fungus growth on the upper leaf surface of some cultivars, or it may be indistinct and confined to the lower leaf surface. Chlorotic spots with reddish borders are common on the leaf surface and may be mistaken for symptoms caused by the red ringspot virus.
Rust Rabbiteye cultivars suffer from premature defoliation that appears to be caused by a rust fungus, tentatively identified as Pucciniastrum vaccinii (Synonym P. myrtillus). This disease also occurs sporadically on highbush blueberries after harvest.
Resistance to leaf spots is variable with bush age and often only partially effective, so fungicides are generally used in combination with resistance to obtain adequate control, as in the case of cultivars like Croatan, Murphy and Jersey. Reveille and Bladen are two relatively new southern highbush cultivars that appear to be very resistant even when unsprayed.
Fungicidal control currently relies on the use of Benlate (benomyl) and Captan, although other materials may be available. Generally, the earlier applications are the most effective, and growers in NC are advised to make at least one fungicide application for leafspot control before harvest on highbush blueberries, more on rabbiteye. The remaining fungicide applications are spaced every 2 weeks from harvest until mid-August. The use of crop oils or Trilogy (Neem Oil) in early season sprays actually increases leaf spots, probably by damaging the protective waxes on the leaf surface that normally block fungal penetration.
mowing (topping) has become common practice in NC as a means of pruning
to maintain proper bush height without having to use hand labor every
year. An added benefit of this pruning technique is the reduction in leaf
diseases by removal of older, infected leaves and subsequent rejuvenation.
New midsummer foliage produced after topping persists well into fall without
serious leafspot problems.