TWIG BLIGHT OF BLUEBERRY

 

Twig blight is a fungal disease which causes a dieback of fruit-bearing twigs on highbush and rabbiteye blueberry bushes. Yield losses as high as 70% have been recorded on susceptible varieties when fungicidal sprays are not used.

Illustrations:

  • Early symptoms of Phomopsis twig blight.
  • Blueberry twig killed by Phomopsis.
  • Fruit rot may be caused by Phomopsis.

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    Symptoms


    Visible symptoms first occur in late February to early March in southeastern North Carolina, soon after the fruit buds reach the green-tip stage. Individual buds turn brown and die, followed by browning and necrosis of bark around the bud as the fungus spreads from the blighted bud into the twig. The disease usually spreads down the twig until most or all of the fruit buds on an individual twig are killed.

    Twig blight can continue to infect fruit-bearing twigs at all stages of development. New infections occur continually as buds open, flower, and produce fruit. The disease stops progressing after killing the twig (6-10 inches), and does not progress further down the stem to infect older wood. The slender leaf-bearing buds which develop lower on the twig are not initial points of infection with this disease, but they may be killed as the infection spreads down from the fruit buds.

     

    Disease Cycle


    The fungus (Phomopsis vaccinii) overwinters in dead twigs infected during the previous year. Beginning in February, spores (conidia) are released from the old infections and dispersed by wind and rain. The largest numbers of conidia are released from budbreak in February through bloom in April; conidia become trapped in opening buds, germinate, and infect the vascular system. After the twig is killed, fruiting bodies called pycnidia are produced just under the surface of the bark. these fruiting bodies exude conidia which can cause additional infections, with conidial production continuing into August.

     

    Control


    Pruning -- During routine winter pruning, careful hand-pruning can be performed to remove infected twigs prior to spore release in the spring. This will decrease the amount of disease inoculum present at budbreak. While this technique may not be practical for large commercial operations, growers with less acreage and home gardeners should benefit from this sanitation method.

    Cultivar Selection -- Careful cultivar selection can greatly reduce the amount of twig blight experienced. The highbush cultivars Murphy and Harrison are highly susceptible to this disease and should not be grown without using a fungicidal spray program. Croatan is moderately susceptible, while Reveille, Cape Fear, Bluechip and Wolcott are relatively resistant. Rabbiteye cultivars are susceptible, especially Delite.

    Fungicides -- Chemical control can be obtained by spraying benomyl (Benlate) 50% wettable powder at a rate of 1 lb/100 gallons (2 tsp/gal). This fungicide should be applied from budbreak through bloom on a 7-14 day interval; benomyl cannot be applied within 21 days of harvest, so spraying should be stopped soon after green fruit begins to size.

     

    Fruit Rot Stage


    Phomopsis vaccinii also causes a fruit rot on blueberry at harvest in North Carolina. Infected berries become very soft and may split resulting in leakage of juice. The cultivar Harrison is extremely susceptible and for this reason is no longer recommended for planting in NC. Most other varieties can be grown with minimal loss due to the fruit rot stage if fruit is harvested in a timely fashion to avoid overripe fruit on the bush. In North Carolina, fruiting bushes should be harvested every 7 days or less.

     

    Cane Canker Stage


    In southern Michigan and northern Indiana, this fungus causes a cane canker which kills entire stems. Symptoms are similar to those of blueberry stem blight, a common disease in the southeastern United States. This disease stage of Phomopsis vaccinii has not been observed to occur in North Carolina.

    Last updated: 27 May 1997
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