Scaffolds 99 index

APPLE SCAB UPDATE

(Dave Rosenberger dar22@cornell.edu & Fritz Meyerfwm4@cornell.edu, Plant Pathology, Highland)

Apple scab ascospore maturity count:

Hudson Valley Lab, March 29: 99.7% immature spores, 0.3% mature spores, no discharge in the shooting tower.

Apple trees are at Silver Tip. Spore maturity this year is much closer to "average" than it was last year. Ascospore maturity counts performed when trees were at Silver Tip last year (26 March 1998) showed 22.7% mature spores compared to only 0.3% for a similar growth stage this year.

However, the delayed spore maturity compared to last year should not be used as an excuse for delaying preparations for the apple scab season. Growers who had scab problems in 1998 must be prepared to start their fungicide program at Green Tip this year because of the high levels of scab inoculum present in the orchard. So check out those sprayers and have everything ready to go! The first scab sprays might be needed by the end of the week in the Hudson Valley.

A RETROSPECTIVE ON APPLE SCAB IN '98 AND SUGGESTIONS FOR '99

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

What Happened Last Year?

Apple scab was unusually severe throughout New York State in 1998. Several growers in the Hudson Valley said it was the worst scab year since 1945. What factors contributed to the severe scab epidemic in 1998 and what is the best approach for controlling apple scab in 1999? Scab problems in 1998 resulted from a series of unusual weather-related events. Problems started with a mild and wet winter that favored early maturation of apple scab ascospores. Trees reached Green Tip in the lower Hudson Valley on March 26. Both tree phenology and spore maturity advanced very rapidly between March 26-31, when maximum daily temperatures were 82, 82, 79, 87, and 88°F. An infection period occurred April 1-2, about a week before most growers were ready to begin spraying for scab. Extended rains and wetting from May 1 to May 11 (5.5 inches rain, 163 hrs wetting) favored severe secondary infection and precluded effective fungicide protection during the critical period around petal fall. Cool and wet weather during late May and June kept scab active. Heavy rains June 12-17 (5.0 inches of rain, 120 hrs wetting) removed fungicide protection and favored fruit infection. On June 30, 2.9 inches rain again removed fungicide protection and left fruit susceptible to more infections during wetting period on July 4-5 and 7-9.

The earliness of the 1998 season can be illustrated by comparing calendar date and predicted apple scab ascospore maturity at the Hudson Valley over the past 14 years (Fig. 1).

Figure 1. Relationships between calendar date and predicted apple scab ascospore maturity (based on degree-day model) at the Hudson Valley Lab, Highland, NY. Heavy lines are used to show spore maturation in 1987, 1990, and 1998.

Ascospore maturity predictions were derived from the degree-day model developed by Gadoury and MacHardy in the mid-1980's. The model uses accumulated degree-days (base 32°F) to predict the percentage of the total season's ascospore load that is ready to discharge. Spore maturation and early-season tree growth were roughly 15 days ahead of "normal" in 1998. Before that, 1987 was the earliest year. Green Tip occurred earlier in 1990 than in 1998, but cold weather following bud-break in 1990 delayed development.

Lines showing ascospore development for the five years from 1992 through 1997 all fall toward the right side of the graph in Fig. 1. The relative "lateness" of the five seasons preceding 1998 may have contributed to problems last year because problems associated with an early wet season had become only faded memories.

What To Do In '99

Clues for controlling scab in 1999 can be gleaned from the success stories of 1998 and by reviewing research on fungicide efficacy.

1. Start Early: The first scab spray must be applied before the first infection period. In 1998, growers who applied contact fungicides prior to the infection period on April 1-2 had relatively little scab throughout the rest of the season. Growers who applied SI fungicides (Rubigan, Nova, Procure) within 96 hours after the first infection period also had good scab control. However, those who delayed SI applications for more than four days beyond the start of the infection period had to fight secondary scab for the remainder of the season.

(Remember that a copper spray applied at green tip to control fire blight also provides about 7 days of protection against apple scab.)

2. Use a Protectant Program: on a 5-8 day schedule from Green Tip until Pink. This is not a good year to use the four-spray SI program in which the first application is delayed until tight cluster. The only appropriate use for the four-spray SI program in 1999 is in orchards where detailed leaf counts were made at the end of last year to document the absence of scab in the block. Many growers discovered last year that SI fungicides do not perform well in a wet year if scab is well established before the first SI sprays are applied. Using protectant fungicides during the early part of the season is also a good resistance-management strategy. In orchards where SI-resistant strains of scab may already be present, potential losses to these SI-resistant strains will be minimized if the prebloom scab control program is based on contact fungicides.

SI fungicides may still be needed at pink, bloom, petal fall, and first cover to control mildew. (As has always been the case, the SI fungicides should be used in combination with a contact fungicide.) When used beginning at pink, the SI fungicides will provide added protection against scab during the period of peak susceptibility fruit scab. If SI-resistant strains of apple scab are present in the orchard, infections of by these strains will be delayed by the pre-bloom schedule of contact fungicides and risks of fruit infection will be significantly reduced.

3. Review What We All Knew about contact fungicides in the time before SI fungicides were available:

• Captan is considerably stronger against apple scab than is mancozeb. In research trials, Captan 50W at 1 lb/100 gallons dilute spray has provided better scab control than any of the mancozeb formulations applied at 1 lb/100 gallons. (Captan 80W and Captec 4L work as well as Captan 50W; adjust the rates accordingly.)

• Full rates (Mancozeb at 2 lb/100 gallons or Captan 50W at 5-6 lb/100 gal) provide excellent scab control when used on a 7-day program. Half rates (1 lb of Captan 50W or mancozeb) may provide control for only 4-7 days during wet weather.

• Half rates of contact fungicides may work prior to Tight Cluster or when sprays are consistently timed to go on just ahead of infection periods. However, recoverage may be needed 4 days later if rain wash-off has occurred.

• Remember that if mancozeb is used at more than 3 lb/A in any spray, then label restrictions require that no mancozeb be applied after petal fall. If more than 3 lb/A of mancozeb is needed to control scab as may be the case with large trees or exceptionally high scab inoculum, consider using mancozeb/captan combinations. For example, using 3 lb/A of mancozeb in combination with 1.5 lb of Captan 50W/A will provide improved control of both scab and black rot as compared to mancozeb used alone.

• Captan and mancozeb can be applied in the rain to control an on-going infection period, but they will not work if applied more than 18 hours from the start of the wetting event. Growers who applied a contact fungicide in the middle of the May 1-11 infection period last year had less scab than those who didn't spray because it was raining.

4. Consider the limitations of your sprayer: Even the best fungicides will not control scab if they are not applied properly. Over the past 15 years, many big old orchard sprayers have been replaced with smaller PTO-driven sprayers. At the same time, many farms have become larger and therefore need to cover more acres per sprayer. This often results in more sprays being applied under windy conditions because there is insufficient time to cover all the acreage during short windows of ideal spray conditions. However, the small sprayers may not be capable of pushing the sprays against the wind. Imperfect coverage may be "good enough" in a light scab year, but it won't provide adequate control when scab pressure is high. Similarly, proper calibration is important for maximizing effectiveness of scab sprays and other pesticides.

In summary, apple scab could be extremely severe in 1999 because of the high levels of inoculum that exist in many orchards. Probably the most important thing than anyone can do to prevent a repeat of scab problems in 1999 is to have sprayers and orchards ready to go when the first fungicide spray will be needed shortly after bud-break. If early-season infections can be prevented, then the remainder of the season should be much easier than it was in 1998.

PLAN FOR POTENTIAL FOR FIRE BLIGHT ON YOUR FARM

(Deb Breth, dib1@cornell.edu Lake Ontario Fruit Program, Albion)

Weigh the risk of fire blight on your farm by asking the following questions (in order of decreasing risk):

• Will there be active cankers present in the orchards this year?

• Did you have fire blight in the orchards last year?

• Was there fire blight infection in the local area last year?

• Did you have fire blight in the local area two seasons ago?

• Are there any of your orchards or hosts in the local area where there was no fire blight during the last two seasons?

The risk of fire blight epidemics starting with blossom blight increases as you get closer to the source of fire blight infections in the area. The closer you are, the higher the risk, and the more stringent the control measures that must be planned and implemented. The risk is also dependent on other factors specific to the orchard:

• Orchard age, vigor, and variety and rootstock susceptibility

• Orchard size and ease of getting good spray coverage

• Is it a high value orchard, or close to one?

Erwinia amylovora bacteria numbers have the capacity to double in 20 minutes at 70°F. In 10 hours, 1 bacterium becomes 1 trillion bacteria. Therefore, the first step in planning for fire blight potential requires reduction in the number and distribution of inoculum sources available for each phase of the disease (blossom blight, canker blight, shoot blight, "trauma" blight). If blossom blight is controlled, but canker blight is allowed to remain in the trees, the resulting oozing bacteria will be a source of infection for shoot blight the remainder of season. Although eradication of the bacteria is impossible, steps for reducing inoculum sources will reduce the disease incidence.

Cleaning Up Old Fire Blight Infections

• Remove wild hosts in hedge rows in surrounding areas, and neglected apple and pear orchards that are susceptible to fire blight.

• Prune out cankers on an annual basis or remove whole trees if cankers are on the main trunk (or try canker surgery).

The cankers with the indistinct margins have the greatest potential for oozing in the spring. Prune while still dormant before bacteria become active and start to move within the tree cambium.

• Canker removal will also serve to remove sources of black rot, white rot, and bitter rot which cause summer fruit rots.

• Removal of prunings -- whole trees and large branches obviously need to be removed from the orchard for the above reasons, but smaller prunings can be raked to the row middles to chop with a flail mower in the spring.

• If blossom blight is controlled but there is canker blight active in the orchard, there is still plenty of inoculum for shoot blight and trauma blight in the summer.

Fertility: Maintain a balanced fertility program; although nitrogen is known to help increase yields, it also results in greater susceptibility to fire blight.

Early copper sprays: Copper sprays do not kill bacteria inside the cankers. They only reduce the efficiency of the bacteria oozing from the cankers to colonize the bark and bud surfaces during the prebloom period.

Additional benefits of early copper: as an early scab protectant, if applied before the first scab infection; nutritional value; can be applied with oil (often recommended on copper labels as an adjuvant). Results (as in improved control) are not always consistent because of variables associated with timing the application. Control depends on how much copper remains on the tree surface relative to when cankers are oozing bacteria. Good spray coverage is critical.

Bacteria will colonize even non-susceptible varieties, so when applying copper to susceptible varieties in an orchard, you must apply copper to the entire orchard. Early copper sprays do not eliminate the need to control blossom blight; copper just reduces the bacterial population before they can multiply during bloom. The risk of phytotoxicity with copper increases as the tree phenology approaches bloom. Do not apply copper in fresh fruit varieties later than 1/4-Inch Green. Fruit russeting is likely on fresh fruit.