Scaffolds 00 index

(Dave Rosenberger dar22@cornell.edu, Plant Pathology, Highland)

Apple scab ascospore counts as determined from squash mounts:

Date	Location	Immature	Mature	Discharged	Tower Discharge
3/28	Highland	0%	0%	0%	0
3/30	Livingston(Columbia Co.)	0%	0%	0%	0
4/03	Highland	98%	2%	0%	215

Ascospore maturity in eastern NY progressed rapidly with warm temperatures over the past weekend. Leaves collected this morning (April 3) provided the first discharge in our shooting tower test. The first commercially important ascospore release generally does not occur until we have at least 15% mature spores as determined with our squash-mount assessments. Thus, the risk of scab infection in commercial orchards remains low throughout the early part of this week and growers in the Hudson Valley can delay fungicide applications at least until mid-week. Spore maturity is likely to reach the 15% threshold by the end of the week if the weather predictions for warm temperatures prove accurate.

(Dave Rosenberger dar22@cornell.edu, Plant Pathology, Highland)

Some apple growers in eastern New York are noting more than the usual number of canker problems in their orchards this spring. The only "cure" for large cankers is to prune them out. However, early season spray programs may reduce the potential for at least one kind of canker disease. Growers and consultants should be aware of early indications that orchards may be at risk for white rot canker. Some adjustments in prebloom spray programs may be appropriate for blocks where white rot lesions have become established.

The two most common apple cankers in New York are black rot canker caused by Botryosphaeria obtusa and white rot canker caused by Botryosphaeria dothidea. Both of these fungal pathogens can also cause fruit rots during summer. However, in northern growing regions (e.g., New York, New England, Michigan) there is no apparent linkage between incidence of the canker diseases and incidence of the fruit rot diseases. Orchard conditions that favor canker development do not necessarily favor development of fruit rot diseases, nor is there evidence that fruit rots contribute to canker development.

During the first three-quarters of the 1900's, the problem of black rot cankers was addressed by numerous researchers in the northeast, whereas white rot canker was considered a "southern" problem. In the 1980's, Dr. Jim Travis and co-workers at Penn State showed that white rot canker was causing tree decline in parts of Pennsylvania. White rot cankers were subsequently found in some Hudson Valley orchards as well.

It is not always easy to tell the difference between black rot and white rot cankers, but there are some characteristic differences. Black rot cankers are usually initiated at pruning cuts, whereas white rot cankers may be initiated in unwounded bark on trunks and scaffold limbs. Black rot cankers have distinct margins, and the bark and cambium within the canker margins are completely killed (Fig. 1).

Fig 1. Typical black rot canker developing around a bench cut.

White rot cankers frequently begin as depressed areas in the bark with less clearly defined margins. Green cambium is often evident beneath superficial white rot lesions (Fig. 2). Bark areas affected for several years may develop large wart-like structures surrounding lenticels (Fig. 3). The superficial white-rot lesions do not seem to cause economic damage until the trees become drought stressed. Under drought stress conditions, the white rot fungus penetrates through the cambium and the resulting cankers suddenly become obvious. In some cases, white rot cankers that develop during summer may ooze, almost like active fire blight cankers.

Fig. 2a. Sunken bark typical of a superficial white rot lesion.

Fig. 2b. Necrosis and necrotic flecking gradually develops beneath sunken lesions.

Fig. 3: Large "warts" may develop in bark areas affected by the white rot fungus.

Field observations in New York suggest that black rot cankers develop primarily on trees where xylem tissue has been sequentially damaged by cold injury and by basidiomycete wood-invading fungi such as Schizophyllum commune and Trametes versicolor. The wood-invading fungi follow saprophytic yeasts and bacteria into pruning cuts where they colonize, discolor, and soften the old wood in the center of trunks and limbs. Healthy trees create chemical and physical "barrier zones" that keep these weak pathogens from invading undamaged xylem, but stressed trees cannot maintain effective barrier zones. The wood-invading fungi advance from the center, damaged wood outward toward the bark until the perimeter of living tissue can barely support life functions of the limb. It is at this stage that B. obtusa moves into the surface tissue and completes the killing of the bark to form a visible black rot canker. Incidence of black rot cankers often increases dramatically three to five years after a severe winter-injury event because it takes three to five years for wood-invading fungi to weaken limbs enough to allow development of a black rot canker.

Development of white rot cankers in New York seems more closely correlated with drought stress than with winter injury. (Drought conditions also allow more rapid extension of black rot cankers, but drought seems less important in the epidemiology of black rot cankers.) White rot cankers are more abundant following drought conditions because superficial white rot lesions on the bark surface are able to completely penetrate the bark only when the tree is under drought stress. Most fruit growers fail to notice superficial white rot lesions on trunks and older limbs because the superficial lesions can be mistaken for a natural part of the "aging process" that occurs as trunks transition from smooth to scaly bark. Nevertheless, I have seen at least three orchards where failure to control superficial white rot lesions over a period of years allowed development of severe canker problems in a drought year.

There is no evidence that spray programs can slow development of black rot cankers under New York conditions. However, spray programs definitely can affect the incidence of the superficial white rot lesions that contribute to development of visible white rot cankers in drought stressed trees. Observations I have made over the past 15 years suggest that superficial white rot lesions gradually increase in abundance in orchards where early-season scab sprays consist of either SI/EBDC combinations or low rates of EBDCs (mancozeb, Polyram) used alone. Neither the EBDCs nor the SI fungicides (Nova, Procure, Rubigan) have much activity against Botryosphaeria species. EBDCs may have been somewhat effective when they were applied at rates of 6—8 lb/A, but they no longer suppress white rot when applied at 3 lb/A. The benzimidazoles (Benlate, Topsin M) controlled this disease when they were used for scab control, but these fungicides are rarely used in prebloom sprays today because the scab fungus is resistant to benzimidazoles in many orchards.

No research has been done to determine when superficial white rot lesions are initiated or the best method for controlling them. Therefore, we have no scientific basis for making control recommendations. However, I suspect that copper sprays applied at green-tip to half-inch green may be the most cost-effective approach for suppressing white rot lesions because the copper residues remain on the bark for an extended period of time. Including Topsin M or Benlate in a tight cluster or pink spray might also help reduce the incidence of superficial white rot lesions. The new strobilurin fungicides (Sovran, Flint) are effective against Botryosphaeria species and may help to suppress white rot lesions if they are applied prebloom.

White rot canker does NOT pose a significant threat for most New York apple growers. No changes in fungicide programs should be needed in orchards with trickle irrigation or orchards where there is no evidence of superficial white rot lesions. However, fungicide adjustments may be warranted in non-irrigated orchards where superficial lesions are abundant, especially considering that the long-term weather forecast is calling for another dry summer.

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