Scaffolds 01 index

(Bill Turechek wwt3@nysaes.cornell.edu, Dave Rosenberger dar22@cornell.edu, & Herb Aldwinckle hsa1@nysaes.cornell.edu, Plant Pathology, and Jim Schupp jrs53@cornell.edu & Terence Robinson tlr1@nysaes.cornell.edu, Horticultural Sciences)

Fire blight remains one of the most destructive and difficult-to-control diseases of apples and pears. Young high-density apple plantings are especially at risk because they often contain vigorously growing, blight-susceptible cultivars that were propagated on highly susceptible rootstocks. Under high risk conditions, the recommended applications of copper at green tip and streptomycin during bloom may not provide complete protection against fire blight. When blight becomes established in young orchards, large numbers of trees can be killed within a single season. This article contains suggestions for optimizing use of Apogee for controlling fire blight in orchards that are at high risk.

Apogee (Prohexadione Calcium) is a growth regulator that has demonstrated potential for managing shoot blight infection in experimental trials conducted in New York, Michigan, and Virginia. Apogee is ineffective for control of the blossom blight phase of the disease and is registered only for apples, not for pears. Apogee works by "shutting down" the growth of a tree and, therefore, is used primarily to control overly vigorous trees and reduce the need for seasonal pruning. Apogee has value in fire blight management because when trees stop growing, they become relatively resistant to new blight infections and further expansion of established infections is arrested. Thus, Apogee can significantly reduce secondary spread of fire blight (i.e., shoot blight infections) in orchards where streptomycin sprays failed to provide 100% control of blossom blight. (Shoot blight is rarely a serious problem in orchards that do not have any blossom blight unless secondary inoculum is coming from adjacent blocks that had blossom blight.) In trials conducted in New York, the best control of shoot blight was obtained when Apogee was applied during late bloom or early petal fall (when shoots were 1 inch long) at 12 oz/ 100 gal, with a second application 3 weeks later.

The problem with using Apogee to control shoot blight is that the first application of Apogee must be made before the effectiveness of streptomycin blossom sprays can be evaluated. Research trials in both the Hudson Valley and Geneva have shown that if the first Apogee application is delayed until blossom blight symptoms appear, then Apogee will have almost no benefit for controlling fire blight. Apogee has no effect on shoot growth or fire blight for at least 10 days after application, so it acts too slowly to be of value as a rescue treatment for orchards with blight symptoms.

In mature orchards where trees have already filled their spaces, the decision on whether or not to use Apogee can be based on a combination of its potential value as a vegetative growth inhibitor and as a supplement to fire blight control. In young orchards where trees have not yet filled their spaces, the decision is much more complex. Using Apogee for fire blight control in young orchards will cause reduced vegetative growth (see Table 1 for an example). That, in turn, will decrease profitability of the orchard in succeeding years because it will increase the number of years required for trees to fill their spaces and for the orchard to reach the break-even point. Because of this, one needs to seriously consider whether the delay to reaching full production and/or the reduction in fruiting capacity outweighs the potential loss due to fire blight plus the cost of an application, and that decision must be made at petal fall.

Table 1. Calculated estimates of tree size and yield in years three through six of 'Gala'/M.26 apple trees treated with Apogee in the third year of planting. Calculations on Apogee-treated trees are shown in parentheses and were calculated based on the assumption that Apogee application would reduce tree canopy (volume) by 40% compared with unsprayed trees.

The cost-benefit analysis for deciding whether or not to apply Apogee in young orchards will hinge on several factors. These include the number of fire blight infection periods that occurred during bloom, the severity of fire blight the previous season, the susceptibility of the scion variety and rootstock, and the age and vigor of a planting. After extended discussions among the fire blight researchers and horticulturists at Geneva and Highland, we developed the following model for determining when Apogee applications might be justified as a blight-prevention strategy in a young apple orchard. This point system was derived from our "best guesses", so this model will undoubtedly change as more data become available. The model assumes that growers are making every attempt to control blossom blight using streptomycin or streptomycin plus Messenger in young orchards. Apogee use is therefore viewed as an additional measure to be employed only in the most severe situations.

At petal fall, circle the appropriate response for the orchard in question:

1. Was fire blight found in or around your orchard in the past 2 years? Yes=1, No=0
2. Is the cultivar susceptible? Very=100, Moderately=50, or Slightly=0
3. Tree vigor status? Below average=0, Average=50, or Above average=100
4. Did MARYBLYT predict a high risk or an infection event prior to petal fall?

No=0,

Yes and total EIP is less than 200=50,
or Yes and total EIP is greater than 200=100

5. Did you omit any streptomycin sprays that would have been required to cover MARYBLYT infection periods? No=0, Yes=100
6. Have there been any trauma events during bloom (hail or wind storms)? No=0, Yes=100

Now apply the formula: y = 1 - (2+3+4+5+6), where you replace the numbers in the formula with the points from the corresponding question above. If y is 250 or greater, then an Apogee application is probably warranted. For Apogee treatment of trees less than 5-yrs old, the rate of application should be reduced to 6 oz/100 gal, and the grower must balance the benefit of shoot blight control against the drawback of reduced shoot growth.

Several questions come up as one considers each of these conditions. Again, we stress that this model was written specifically for plantings between the ages of 2 to 6 year old, the orchard ages that frequently suffer the most severe tree losses to fire blight. Fire blight in or around your orchard means exactly that. In this model, we make the assumption that disease pressure is insignificant in orchard blocks where fire blight has been absent for two or more years. We also assume that the orchard blocks in question are regularly scouted and that good sanitation practices, such as pruning, are followed. If any of your blocks (or any of your neighbor's blocks) have had fire blight in them in the last two years, consider that the potential for fire blight infection exists in your orchard should conditions be favorable during bloom. Favorable conditions means those conditions which pose a high risk for infection as determined by the fire blight forecaster MARYBLYT.

Apple cultivars differ considerably in their susceptibility to fire blight, but cultivars may be grouped differently, depending on how susceptibility is evaluated. Table 2 ranks the susceptibility of several cultivars based on field observations. Although a particular cultivar's susceptibility to fire blight is largely a function of its genetic make-up, other factors, particularly nutrition, contribute to a tree's susceptibility. In general, vigorously growing trees are more susceptible to fire blight than slow-growing trees. Orchards with high nitrogen levels where young trees are being pushed to fill their allotted space should be considered high risk for fire blight and thus good candidates for Apogee to slow tree growth.

Table 2. Relative susceptibility of commonly grown cultivars to fire blight.¹

¹ Modified from: Breth, D.I., Reddy, M.V.B., Norelli, J., and Aldwinckle, H. 2000. Successful fire blight control is in the details. New York Fruit Quarterly 8(1):10-16.

For those who run MARYBLYT on their farm, the answers to question numbers 4 through 6 are straightforward. Referring to question 4, consider the highest EIP (Epiphytic Inoculum Potential) value attained during bloom. Remember that MARYBLYT resets EIP values to zero after a streptomycin application, so be careful to look over the entire range of EIP values during bloom. Referring to question 5, a missed application is an application that was not applied within 24 hours after a MARYBLYT 'high risk' warning (i.e., 3 out of the 4 infection criteria have been met). We assume that you check and update MARYBLYT once every 24 hours (i.e., daily, preferably at the same time each day). And lastly, we consider a trauma event has occurred if hail has opened wounds on trees or winds have been high enough to noticeably damage the foliage prior to petal fall. Under the most severe conditions (e.g., those that occurred in Southwestern Michigan in 2000), virtually all apple cultivars can develop fire blight. This is a judgement call and should vary depending upon the varieties affected and the extent of damage.

Apogee is used in situations where fire blight was not well managed (for whatever reason). It is important to note that Apogee will not and cannot act as a substitute to standard management practices. Thus, an effective program for management mandates that Apogee be used in conjunction with a delayed dormant copper spray, timely blossom blight sprays, and good sanitary practices as part of an integrated pest management strategy to control fire blight.

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

In New York, there are two economically important diseases of cherry that require attention at petal fall.

Cherry leaf spot is one of the most serious diseases of cherry. The disease affects primarily the foliage of both sweet and tart cherry. In general, tart cherries are much more susceptible to the disease than sweet cherries. Severe infections can result in total defoliation by midsummer, which weakens the trees and makes them more susceptible to cold injury. If heavy infection occurs early in the season, the fruit will fail to develop. Infections can occur anywhere from petal fall to harvest when conditions are suitable.

The disease is caused by the fungus Blumeriella jaapi. The fungus survives the winter in infected leaf tissue that falls to the ground. In early spring, a fruiting structure is produced that contains the primary spores that initiate disease. These spores begin to mature around bloom and, for several weeks afterwards, are released during periods of rain or any significant wetting event such as heavy dew. The spores invade the plant through stomata, which are found primarily on the underside of the leaf, on leaf and fruit stems, and for a short period, on fruits. Immature leaves are resistant to infection because they have yet to unfold and expose their stomata. In fact, infection in early spring is somewhat limited because the availability of susceptible tissue is at a minimum during the primary period of spore release.

Once infection has occurred, symptoms appear in approximately 10—14 days. The symptoms begin as small, circular red to purplish spots on the upper leaf surface. Spots enlarge as they grow older, typically coalescing and turning brown, and may eventually drop out of the leaf to give the leaf a "shot-hole" appearance. The most striking symptom of cherry leaf spot, especially on tart cherry, is the yellowing of older leaves prior to their falling from the tree. When infection is severe, the entire tree may be defoliated by midsummer. Numerous spores are produced on the underside of leaf lesions and appear as a white to pinkish mass in the center of the lesion. These spores are moved throughout the orchard by wind and rain and are responsible for driving the epidemic.

The severity of infection is dependent upon the length of the wetting period as well as the temperature during the wetting event. Table 11 in Cornell's Pest Management Guide for Commercial Tree Fruit Production shows the predicted degree of infection relative to the average temperature and the number of hours of leaf wetness. Under the optimum conditions, light infection can occur with as little as 5 hours of leaf wetness when the average temperature is at the optimum of 65°F, but nearly 20 hours of leaf wetness are needed when the average temperature is less than 50°F when conditions are less than optimal.

The only practical way to manage this disease is through the use of fungicides. Fungicide applications should begin at petal fall and continue on a 7—10-day schedule until harvest. Calendar sprays can be avoided by timing applications prior to infection events (see Table 11). Elite, Indar, and Rubigan have approximately 3 days of kickback activity and can be used in this mode if necessary. Bravo has the longest residual activity and also has activity against black knot.

Powdery mildew of cherry is caused by the fungus Podosphaera clandestina. The fungus attacks young leaves and shoots and, like leaf spot, is a bigger problem on tart cherry than sweet cherry. The symptoms of the disease appear as a white powdery growth on either side of the leaf or on the terminal ends of the shoots. Severely infected leaves curl upward and eventually drop as they age. Temperatures between 65—75°F, with little rain, but with sufficient humidity, such as those caused from heavy dew or fog, are the most conducive conditions for rapid disease development. Powdery mildew is one of the most severe diseases of cherry in the Pacific Northwest. However, the disease is typically not of economic significance in New York orchards. Where the disease is a problem, Nova is the most effective fungicide, and applications should begin shortly after petal fall.

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