Fruit Rot Diseases of Blueberry
W.O. Cline, Extension Plant Pathologist
Fruit rot diseases of blueberry are usually caused by fungi. These fungi are carried over from one season to the next on dead or infected plant parts (stems, twigs, leaves and fruit). Fruit of the current season's crop can be infected at any time after bloom, but infections often are not visible until berries begin to ripen. At harvest, infected berries may be soft and leaky, or may have masses of fungal spores growing on them. These spores can spread to adjacent ripe, healthy berries and cause them to rot after harvest. Postharvest decay is a serious problem in marketing channels on fruit from all major production areas in the United States.
Symptoms and Disease Cycles
The fungus survives the winter in blighted twigs. Spores are released from blighted twigs throughout the growing season during rainy periods to initiate latent infections on immature green fruit. These fruit remain symptomless until maturity. At harvest or soon after, the infected fruit (Figure 1) rapidly decay and exude masses of orange spores capable of starting new infections. The fungus primarily rots fruit, but leaf infections can occur. Leaf symptoms vary from small brown circular to irregular shaped lesions to large, black, poorly defined necrotic lesions. Losses in North Carolina from preharvest rots caused by C. acutatum are estimated at 1-5 percent while losses from postharvest decays are probably much higher on susceptible cultivars.
Alternaria rot (Alternaria tenuissima) is primarily a postharvest disease of blueberry, although rots do occur in the field when fruit remains on the bush beyond normal ripening. Studies of fruit rot diseases conducted following storage or shipment of fruit show that this disease is the most damaging and most commonly occurring postharvest rot of blueberry.
The disease overwinters in old, dried-up berries and in dead fruit-bearing peduncles on bearing twigs from the previous season's crop. The pathogen causes sunken lesions (Figure 2) on berries when infection occurs prior to harvest. The surface of the infected area is often covered with a greenish-black spore mass, making it easy to differentiate from the orange spore mass on a ripe rot-infected berry. Alternaria spores can be splashed or carried by wind-blown rain to other berries. In post-harvest experiments, 96% of alternaria rot infections occurred through the stem scar of the berry. This indicates that most alternaria infections are not initiated until after fruit is harvested, because the stem scar is only exposed when berries are detached. Postharvest infections produce fuzzy mycelial growth on the surface of the berries.
Phomopsis soft rot (Phomopsis vaccinii) is a fruit rot caused by the same fungus that causes blueberry twig blight in NC and blueberry cane canker in northern states. This fruit rot disease was relatively unknown and of minor importance until the release of the cultivar Harrison. Although high yielding and large fruited, Harrison is extremely susceptible to Phomopsis fruit rot and for this reason is no longer recommended for planting in commercial production fields. The fungus overwinters in twigs infected during the spring twig blight phase of this disease (for more information, see Fruit Disease Information Note No. 10, Twig Blight of Blueberry). Spores from infected twigs are carried by rain to developing berries, where they infect and cause a soft rot at harvest. This soft rot is often only detectable by feel, and by the presence of berry juice in the picking bucket (leaky berries).
Mummy berry caused by Monilina vaccinii-corymbosi is undoubtedly the most damaging fruit-infecting disease that occurs on highbush blueberry at harvest in NC. Prior to harvest, infected berries (Figure 3) become light cream-color rather than normal blue and drop to the ground. These infected fruit, if left on the ground, form overwintering sclerotia and provide a source of inoculum the following year.
Abiotic factors can significantly reduce fruit quality, and affected berries are often mistakenly assumed to be infected by some devastating biotic disease. These factors include heat, excessive handling, wet handling, splitting caused by extended rainy weather during harvest, freeze injury, chemical injury (Figure 4) and improper harvest intervals.
Fruit rot diseases
are best controlled by using several integrated strategies. No one control
method is sufficient, i.e. fungicide use will not make up for poor handling
or a lack of postharvest cooling. Successful growers are those who use all
the tools available to them to produce and maintain a quality berry.
Resistance. Cultivar selection is critical for commercial production. As mentioned, some cultivars such as Harrison and Cape Fear are very susceptible and are no longer recommended. While most cultivars can be grown successfully in homeowner or pick-your-own plantings, commercial fields should not be planted without consulting your county agent or horticultural specialist for advice. Physical characteristics of some cultivars may also predispose them to disease. Large, wet scars at the point of attachment of fruit to the pedicel (stem scars) are major avenues of entrance for fruit-rotting fungi, accounting for 90% of postharvest decay. Emphasis should be placed on the use of cultivars with dry stem scars, which reduces postharvest contamination/infection even when berries are exposed to spores.
Fungicides labeled for use on blueberry can be benefical in controlling fruit rots when applied beginning at full bloom and continuing for 2-3 semiweekly applications. Captan is the most frequently used fungicide for fruit rots and one of the few that can be used close to harvest. Ripe rot is the disease most often reduced by fungicide applications in NC, while little effect is seen on alternaria rot.
Sanitation/Cultural Practices. Harvesting to remove all ripe fruit from the bush every 7 days (or fewer) will drastically reduce fruit rots in the field that would otherwise contaminate the harvesting and sorting process. Many opportunities for fruit contamination occur during harvesting and sorting. Juice from overripe, bruised and split fruits leaks onto other fruits, field containers, and sorting equipment. Decay-causing spores from diseased berries contaminate the juice. Every healthy berry that comes in contact with the contaminated juice on berries or equipment surfaces can also be infected with the decay-causing organism. Prevention of infection through proper sanitation is an effective method for controlling fruit decay. Once infection has occurred the fungus grows into the fruit very rapidly and begins breaking down the fruit tissue. The fungus cannot be reached by currently labeled chemical dusts or sprays after it has entered the fruit. Any treatment drastic enough to destroy a fungus within the fruit usually will also destroy the fruit. Any equipment or containers that come in contact with the fruit must be kept clean. A solution of 10 percent laundry bleach (5 to 6 percent sodium hypochlorite) in water will disinfest the equipment and containers, but equipment and containers must then be allowed to dry since the bleach solution cannot be used in contact with berries.
Cooling has given the most consistent control of postharvest decay.
Cooling the fruit after harvest retains quality and prolongs shelf life.
Cool as quickly as possible to 40o F (5o C) or lower,
but not below 32o F (0o C). If cooled promptly and
kept cool, quality blueberries packaged ready for retail sale can be expected
to hold up well at 32o F (0o C) for 2 weeks and
at 40o F (5o C) for 1 week, but only for 2 days
at 70o F (21o C). Forced-air cooling is the most
satisfactory method for quickly reducing the temperature of palletized
blueberry fruit in consumer-ready containers. For additional information
on cooling blueberries, see NCCES publication nos. AG-413-7, Postharvest
cooling and handling of blueberries, and AG-414-3, Forced-Air
updated: 29 May 1997