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Tree Fruit: Pathology Research Projects

Management of Diseases of Tree Fruits

Management of Diseases of Tree Fruits

Problem: Ten or more fungal diseases annually impact the $235 million value of Virginia's apple industry. On-going fungicide evaluations are essential to disease management in view of the development of resistance to current materials and potential loss of older, useful materials through withdrawal of registration. The sterol-inhibiting (SI) fungicides have been valued in the past for after-infection control of scab, rusts and powdery mildew; however, the relative commercial value of the SIs is being compromised by development of resistance in the scab fungus in some areas of Virginia. The potential for resistance by several important diseases to several fungicide classes, the need for control of numerous diseases, and varying annual disease pressure underscore the need for ongoing testing of new materials and novel approaches for economical, environmentally sensitive disease management. Resistance to fungicides brings shifts in usage patterns and changes in prominence of diseases. We have seen a recent increase in mildew, and Alternaria leaf blotch has spread to new areas of Frederick County in 2006-08.

Response: Since 2007 we have explored approximately 20 new products and mixtures for control spectrum, economics, compatibility and suitability as mixing partners in the commercial spray program. This involved 18 experimental compounds for broad-spectrum fungal disease control and resistance management on apples. Scab fungus population monitoring plots involving several fungicide classes are established where resistance to the SI fungicides was first found. Testing of scab isolates on agar, conducted by Ph.D. student S.C. Marine (pictured above) in cooperation with Dr. D.G. Schmale III, confirmed that isolates from SI-treated trees were less inhibited than those collected from non-treated trees. Under moderate scab pressure, the strobilurin compound trifloxystrobin performed better than myclobutanil for scab control but we have now collected scab isolates for testing of sensitivity to the strobilurin class as well.

Several combination products or tank-mixes show promise for management of SI-resistant scab and other important diseases. Pristine, a mixture of two chemical classes, controlled scab and flyspeck, and also sooty blotch, mildew and several fruit rots. Ten experimental SI fungicides have given expected control of rusts and mildew, but two of them, difenoconazole and fenbuconazole, were also effective on sooty blotch and flyspeck. An experimental package-mix of trifloxystrobin and fluopyram has given excellent control of scab and mildew and is outstanding in its reduction of overwintering mildew inoculum, but had a relative weakness for quince rust control. Two developments confirm potential for improved control of postharvest rot problems: Pristine, applied in the orchard in the late cover sprays, has improved postharvest control of bitter, white, and Alternaria rots as well as Penicillium blue mold. Fludioxonil (Scholar) applied after harvest, suppresses latent bitter rot and controlled blue mold.

Impact: High levels of fungicide resistance in apple scab populations indicate that replacement programs should be considered. The potential for resistance by several important diseases to several fungicide classes, the need for economical management of ten or more diseases, and varying disease pressure from year to year across fruit-growing regions of Virginia underscore the need for ongoing testing of new materials and novel approaches for economical, environmentally sensitive tree fruit disease management. Ongoing fungicide evaluations are essential to tree fruit disease management in view of the development of resistance to current materials and potential loss of older, useful materials through withdrawal of registration. A long-term goal of the resistance monitoring research funded by Virginia Apple Research Program (VARP) and Virginia Agricultural Council (VAC) is to direct it toward DNA-based methodologies so that we can rapidly screen for resistance and more quickly employ the appropriate disease management strategies. This project relates to Virginia Tech College of Agriculture and Life Sciences' key initiative in infectious diseases and Virginia Cooperative Extension's planned program in pest management.


Fire Blight

Fire Blight

Fire blight is a bacterial disease (caused by Erwinia amylovora) that attacks apples, pears and several rosaceous ornamental species. It is a major ongoing concern in apple production because of the serious threat to susceptible apple rootstocks (M.9, M.26 and Mark) and cultivars now being planted, including Gala, Fuji, York, Idared, Pink Lady and Rome Beauty. Typically the bacteria overwinter in cankers and initial infection occurs during warm, wet weather at bloom. Hail or violent storms frequently trigger secondary infection of late bloom and shoots in late spring and early summer. A 100-acre orchard in Virginia lost more than 2,500 Red Delicious/M.26-rooted trees to fire blight following a single blossom infection event in 1991.

Blossom Blight Research: From 2001-2005 we conducted eleven fire blight tests of 104 treatments on several apple cultivars. These involved several management approaches: several registered and experimental approaches for blossom blight suppression including the antibiotics gentamicin and streptomycin, copper formulations and other fungicides, and several biocontrol agents. The biocontrol agents, tested with support of the IR-4 Biopesticide Program included BlightBan and Serenade but were generally more variable and less effective than streptomycin in our tests. Based on our recommendations, most growers are now applying copper materials at green tip stage to help reduce fire blight inoculum pressure and to offset the potential for development of resistance to streptomycin (the most effective material for fire blight control unless resistance develops). So far, we are not aware of resistance to streptomycin in Virginia and we hope to avert that possibility by careful management practices. We encourage growers and consultants to monitor weather and bloom conditions and use a fire blight predictive system (MaryBlyt) on which to base their need for protective sprays. We post Winchester area fire blight infection predictions to this web site (see current season or archived disease updates).

Shoot Blight Suppression Research: In 1993 we initiated a novel means of suppressing shoot blight susceptibility using plant growth regulators (PGRs). This culminated in the first federal registration of a plant growth regulator (reduced risk compound prohexadione-Ca, Apogee) to reduce susceptibility of shoots to fire blight. In research/demonstration plots, funded by the industry sponsored VA Apple Research Program (VARP) in 2001-02, Apogee gave 85% suppression of shoot blight strikes when applied at petal fall on king bloom in nine tests in commercial orchards. Hail injury, in the three-week period following bloom, was a factor in secondary infection in all of these tests. More recent studies supported by VARP have focused on integrating streptomycin and Apogee applications for commercial control. Results of our fire blight research have been published in HortScience, Biological and Cultural Tests, and Fungicide and Nematicide Tests, and Compact Fruit Tree.


Apple powdery mildew

Apple Powdery Mildew

Most tree fruit fungal diseases are favored by wet weather, but powdery mildew is the only fungal apple disease that is capable of infecting without wetting from rain or dew. Because growers tend to be more concerned about control of wet weather diseases, mildew can be a persistent disease of susceptible apple cultivars throughout the mid-Atlantic region. Symptoms of mildew are fruit russetting (shown in image above), whitish lesions on curled or longitudinally folded leaves, and stunted whitish-gray twig growth evident on dormant shoots. Economic damage occurs in the form of reduced yield due to poor return bloom and yield of bearing trees, aborted blossoms, reduced fruit finish quality, reduced vigor, and stunted growth and poor form of nonbearing trees. Mildew severity and the need for control measures are related to cultivar susceptibility and intended fruit market.

Disease Cycle: The fungus overwinters mostly as mycelium in dormant blossom and shoot buds produced and infected the previous growing season. Conidia are produced and released from the unfolding leaves as they emerge from infected buds at about tight cluster stage. Conidia germinate in the high relative humidity usually available on the leaf surface at 50 to 77 °F with an optimum of 66 to 72 °F. Germination does not occur in free moisture. Early-season mildew development is affected more by temperature than by relative humidity. Abundant sporulation from overwintering shoots and secondary lesions on young foliage leads to a rapid buildup of inoculum. Secondary infection cycles may continue until susceptible tissue is no longer available. Since leaves are most susceptible soon after emergence, infection of new leaves may occur as long as shoot growth continues. Fruit infection occurs from pink to bloom. Overwintering buds are infected soon after bud initiation.

Linked Publications

Other Publications

  • Yoder, K.S., A.E. Cochran II, W.S. Royston, Jr., S.W. Kilmer and J.E. Scott. 1998. Integrated fungicide schedules for suppression of powdery mildew and other diseases on Idared apple, 1997. Fungic. Nematic. Tests Vol. 53:43-44.
  • Yoder, K.S., A.R. Biggs, R.K. Kiyomoto, R. McNew and D.A. Rosenberger. 1997. Foliage susceptibility of 23 apple cultivars in the NE-183 trial to scab, powdery mildew, cedar-apple rust, and leaf spots, 1996. B&C Tests 12: 42-43.
  • Yoder, K.S., and K.D. Hickey. 1995. Apple powdery mildew. Pp. 101, 218, and 238 in H.W. Hogmire, Ed. Mid-Atlantic Orchard Monitoring Guide. Northeast Reg. Ag. Engineering Service. 361 pp.
  • Yoder, K.S., R.E. Byers, A.E. Cochran II, W.S. Royston, M.A. Stambaugh, and S.W. Kilmer. 1994. Evaluation of scab-resistant apple cultivars for cedar-apple rust and mildew susceptibility, 1992-93. B&C Tests 9:11.
  • Yoder, K.S. 1992. Powdery mildew of apple. Pages 66-89. In: J. Kumar, H.S. Chaube, U.S. Singh, and A.N. Mukhopadhyay (eds.) Plant Diseases of International Importance, Vol. 3. Diseases of Fruit Crops, Prentice Hall, New Jersey. 456 pp.
  • Hickey, K.D. and K.S. Yoder. 1990. Apple powdery mildew. Pages 9-10 in A.L. Jones and H.S. Aldwinckle, ed. Compendium of Apple and Pear Diseases. Am. Phytopathol. Soc., St. Paul, MN. 100 pp.
  • Yoder, K.S., A.E. Davis, and B.A. Hadley. 1986. Methods for monitoring resistance to benomyl in Venturia inaequalis, Monilinia spp., Cercospora spp. and selected powdery mildew fungi. Pp. 73-75 in K.D. Hickey, ed. Methods for Evaluating Pesticides for Control of Plant Pathogens. Am. Phytopath. Soc., St. Paul, MN. 312 pp.
  • Yoder, K.S. and K.D. Hickey. 1983. Control of apple powdery mildew in the mid-Atlantic region. Plant Disease 67:245-248.