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(Bill Turechek wwt3@nysaes.cornell.edu, Plant Pathology, Geneva)

Last year's particularly wet season set the stage for the appearance of Phytophthora root and crown rot this year. Unseasonably wet weather last year created conditions that were very suitable for the development of Phytophthora spp., the fungal pathogen responsible for causing the disease. This allowed for the initiation of several new infections and the progression of established infections. The disease is now becoming apparent as the summer progresses and temperatures heat up, and plants become stressed as a result of their maturing fruit load.

Apple, cherry, peach, and apricot trees are usually more susceptible to infection than are pear and plum trees. The symptoms on trees usually develop over several seasons, becoming progressively worse over time. The rate of disease development is dependent upon the inherent susceptibility of the variety/rootstock, environmental conditions, the degree of fungal infection, and the overall physiological and nutritional health of the tree. Disease symptoms may become noticeable in early spring as delayed bud break and possibly tip dieback. These symptoms are not a result of direct infection at these points, but are characteristic of a plant under stress. Often, these early symptoms may not appear or simply pass unnoticed. Infected trees often have a normal bloom, giving a false impression of good health.

One picture may be found here: http://www.nysipm.cornell.edu/factsheets/treefruit/diseases/phyt/phyt.fig2.jpeg
and another here:

http://www.nysipm.cornell.edu/factsheets/treefruit/diseases/phyt/phyt.fig3.jpeg

However, developing fruits typically remain small, leaves begin to wilt and drop, and the tree shows a general decline. The decline generally progresses until the trunk is girdled and the tree dies.

It should be noted that the general decline and wilting of trees associated with Phytophthora infection can be associated with a variety of causes other than Phytophthora. Rootstock blight (caused by fire blight), "wet feet" (root asphyxiation), borers, winter injury, graft union necrosis (tomato ringspot virus), etc. are often misdiagnosed as Phytophthora. To distinguish Phytophthora from these other possibilities is not always so simple. Diseased tissue often shows a characteristic reddish-brown discoloration of the inner bark several inches below the soil line (where the fungus first enters the tree). Also characteristic is a clear-cut margin of diseased as opposed to healthy tissue. Aside from the diagnostic symptoms, the only other means of positively diagnosing the disease is to isolate and culture the pathogen in the laboratory; this may take several weeks.

Disease Management. Successful control of Phytophthora can be accomplished through a combination of cultural and, when necessary, chemical practices. The most important factor in disease management is choosing and preparing your planting site. Sites that drain poorly, are slow to dry, and/or experience periodic flooding should be avoided. In many cases, marginal planting sites can be greatly improved with the installation of drain tiles and water-management ditches. The fungus needs standing water to infect. Planting trees on berms or ridges, particularly stone fruits, is highly recommended because it raises the crowns of the tree above the portion of soil where pathogen activity is the greatest. For example, in a berm that stands 4 inches above a flooded orchard floor, fungal activity is reduced 90%; at 10 inches above the flooded floor the fungus is virtually inactive.

In New York, the proper selection of rootstock and variety is perhaps as important as proper site selection and preparation. Apple rootstocks, as well as plum, peach, and cherry rootstock, vary tremendously in their susceptibility to Phytophthtora. Among the apple rootstocks, seedlings, M.9, M.2, M.4, and M.111 are the most resistant; M.7, M.26, and MM.111 are moderately susceptible; and MM.106 and MM.104 are very susceptible.

Among the stone fruit rootstocks, the plum rootstocks 'Myrobalan' and 'Marianna' are relatively resistant. Most peach rootstocks used in production are seedling rootstocks and are therefore genetically variable and lack uniformity in their performance for many traits, including disease resistance. 'Lovell' and 'Halford' are commercially available seedling rootstocks and are considered susceptible to Phytophthora. Little information is available about 'Bailey', an increasingly popular peach stock in the Great Lakes region. 'Myrobalan' and 'Marianna' are also used in peach propagation, and although they may confer Phytophthora resistance to the tree, the grafts do not always produce a horticulturally desirable tree. Among the cherry rootstocks, 'Mahaleb' is the most susceptible. 'Mazzard' and 'Colt' are more resistant and would be recommended for heavier soils. The Gisela series rootstocks (Gisela 5, Gisela 6, Gisela 7, and Gisela 12) are semi-dwarfing to dwarfing rootstocks and have only recently become commercially available. In test plantings, the rootstocks appear to be relatively resistant to Phytophthora. The MxM series rootstocks like MxM2, MxM60, and MxM14 are becoming more important as a rootstock for both sweet and tart cherries in the Finger Lakes region, especially where greater tree vigor is sought. Better knowledge of their susceptibility or resistance to Phytophthora will be obtained as they become more widely planted.

The effectiveness of chemical control is dependent on how far the disease has advanced, the condition of the planting site, and the inherent susceptibility of the tree. Trees that show marked symptoms or are in a severe state of decline typically cannot be revived and should be removed. Trees that are planted in sub-optimal sites, i.e., where disease pressure is likely to occur every year, may be good candidates for chemical treatment, depending on the rootstock. Trees, however, that show mild symptoms, or healthy trees that neighbor declining trees AND are planted in a good site will most likely benefit from fungicide treatment. These trees may be saved or protected from infection when fungicide is applied according to label instructions.

The most effective fungicide for the management of Phytophthora crown and root rot is Ridomil EC. Apply Ridomil EC where crown rot has been a problem or in areas of the orchard where marginal drainage and rootstock susceptibility is likely to be a problem. For apples, make a solution containing 1/2 pt (8 fl oz) of Ridomil EC in 100 gallons of water and apply the solution at the rate indicated in the table below. Applications are made just as growth begins in the spring and immediately after harvest. On new apple plantings, delay the first application until 2 weeks after planting.

On stone fruit, Ridomil applications should be made just before growth starts in the spring and at 2-3-month intervals thereafter if soil conditions are very wet. Apply 2 qts. per treated acre (1.5 fl oz/1000 sq. ft.) in sufficient water carrier to obtain thorough coverage of the soil under the canopy of the trees (material is moved into the soil by subsequent rains or irrigation). Up to three applications can be made per year. On new plantings, delay the first application until 2 weeks after planting and continue at 2-3-month intervals as described above.

References

Biggs, A.R., Hickey, K.D., and Yoder, K.S. Crown or Collar Rot, Phytophthora cactorum. http://www.caf.wvu.edu/kearneysville/

Wilcox, W.F. 1992. Phytophthora Root and Crown Rots. IPM Fruit Crops Disease Identification Sheet No. 7. New York State Agricultural Experiment Station, Cornell Cooperative Extension. http://www.nysipm.cornell.edu/factsheets/treefruit/diseases/phyt/phyt.html

 

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