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APPLE SCAB UPDATE

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

Leaves collected by Kevin Iungerman on April 25 show that the Saratoga/Champlain Valley areas are still lagging other areas in the state vis-a-vis scab ascospore maturity. The Saratoga/Champlain Valley samples show that scab in those areas is now approaching or has just passed our action threshold of 15% mature spores. However, the low counts in the shooting tower suggest that it will take a few more days of warm weather or perhaps some rain to get leaves wet enough to allow maturation to proceed before any major discharges will occur. The slower maturation of ascospores in the Saratoga/Champlain region compared with the Hudson Valley/Western N.Y. regions has been quite striking this year. The last several weeks of dry weather apparently arrested spore development in northeastern N.Y. before spores were at threshold levels, whereas the southeastern and western parts of the state had already passed threshold levels before the dry weather began.

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

Fire blight is perhaps the most devastating disease of apple worldwide. Last year, extensive losses occurred across much of the Midwest and Northeast because weather conditions during bloom, coupled with bouts of severe weather later in the season, created conditions that were extremely favorable for disease development. Add to this that our new varieties, rootstocks, and planting systems have evolved in such a way that newer plantings are more susceptible to fire blight than ever before, and we now have a situation that makes it increasingly difficult to manage this disease. The bottom line is that fire blight is on almost every apple grower's mind. The following article provides an overview of the fire blight disease cycle and discusses management options through the blossoming and post-bloom periods.

Definitions

• Blossom blight starts in spring when flowers become infected. The blossom blight phase of fire blight includes shoot death that develops as a result of bacterial invasion from the flower clusters.
• Shoot blight develops from secondary infections that originate on young terminal shoots. Shoot blight usually develops in late spring or early summer.
• Trauma blight is a term used to describe infections that occur when blight is initiated at leaf or bark injuries resulting from hail or severe windstorms.
• Rootstock blight occurs when bacteria from infected blossoms or shoots moves internally through symptomless trunks and infects roots. Trees on M.26 and M.9 are often, but not always, killed when the roots become infected

Disease cycle

Fire blight is caused by the bacterium Erwinia amylovora. E. amylovora overwinters in cankers on infected limbs. Cankers become active in early spring as temperatures warm and buds begin to develop. Active cankers produce a yellowish to white bacterial ooze that can appear several weeks prior to bloom. During this period, insects (mainly flies) disseminate the bacteria throughout the orchard. During bloom, pollinating insects rapidly move the pathogen from flower to flower initiating the blossom blight phase of the disease. Flowers can become infected within minutes after a rain or heavy dew when the average daily temperatures are equal to 60°F or greater. Flower receptacles and young fruits are resistant after petal fall. Early symptoms of blossom blight can be expected 5 to 30 days after infection depending upon daily temperatures.

Inoculum produced from infected blossoms is further spread by wind, rain, and insects. Shoot tip infections are likely to occur when shoots are actively growing and daily temperatures average 60°F (16°C) or more. In years when blossom infections do not occur, the primary sources of inoculum for the shoot blight phase are the overwintering cankers. Particularly, young water sprouts near these cankers become infected as the bacteria move into them systemically from the canker margins. In the absence of blossom infections, the development of shoot blight infections is often localized around areas with overwintering cankers.

Rootstock blight is associated primarily with the highly susceptible rootstocks such as M.26 and M.9. On these trees, just a few blossom or shoot infections on the scion cultivar can supply bacteria that move systemically into the rootstock where a canker may develop and girdle the tree. Trees affected by rootstock blight generally show symptoms of decline and early death by mid- to late season. Sometimes symptoms may not be apparent until the following spring.

Although mature shoot and limb tissues are generally resistant to infection by E. amylovora, injuries caused by hail, late frosts of 28°F (-2°C) or lower, and high winds that damage the foliage breach the normal defense mechanisms in mature tissues. Instances of fire blight that originate with infections at sites of injury are called trauma blight and may affect even normally resistant cultivars like 'Delicious'.

Disease Management During Bloom

Managing blossom blight is achieved through well-timed chemical sprays. The level of control is critically dependent upon which product you choose to use and the timing of your sprays. The number of applications is typically far less important, per se, than when sprays are applied.

Streptomycin: Streptomycin applications during bloom are highly effective against the blossom blight phase of the disease. These sprays are critical because effective early season control often prevents the disease from becoming established in an orchard. Predictive models, particularly MARYBLYT and Cougar Blight, help to identify potential infection periods and improve the timing of streptomycin, as well as to avoid unnecessary treatments, particularly during the blossom blight phase of the epidemic.

Streptomycin applications are best used in a preventive mode, just prior to an infection event. Using predictive models (e.g., MARYBLYT), it is possible to use local weather forecasts to predict (i.e., guess) whether an infection event is likely to occur in the next day or two. This can be extremely helpful in identifying unusually high-risk situations. In younger orchards, removing blossoms by hand will reduce the risk of blossom infection. This practice can be especially effective in minimizing losses due to rootstock blight as well, particularly when highly susceptible varieties such as 'Gala' or 'Gingergold' are grafted on to M.9 or M.26. Although somewhat time consuming, blossom removal is a much less expensive alternative than replanting an entire block.

Messenger (Harpin) is a unique pesticide that may prove useful for managing both the blossom and shoot blight phases of fire blight. Messenger was recently labeled for use in New York (see related article in this issue). The active ingredient in Messenger is a protein derived from the bacterium Erwinia amylovora (the causal agent of fire blight). The protein is called harpin. Messenger has no direct effect on the viability of the pathogen. Instead, Messenger activates natural defenses within plants to make them more resistant to diseases and physiological stresses. Plants require 5—7 days for full induction of resistance, so Messenger must be applied several days prior to fire blight infection periods. This is a problem with using this compound because it means that the product must be applied before anyone can tell whether or not weather conditions during bloom will actually allow blight infections to occur. The blight suppression provided by Messenger will last for approximately 14 days. In experimental orchards, Messenger applied 10 days before pink and at pink significantly reduced blossom blight, but it was less effective than streptomycin.

Properly timed applications of streptomycin during bloom should still be used as the primary defense against fire blight. Messenger may prove useful as a supplement to streptomycin for situations where blight is expected to be unusually severe either because of high carry-over inoculum in young highly susceptible orchards or when severe blossom blight conditions can be expected based on long-term weather forecasts. We do not yet have enough information to justify recommending routine use of Messenger except in locations where streptomycin-resistant strains of the fire blight bacterium are present. Streptomycin resistance, however, has not been documented in New York. Messenger applied after bloom at 14-day intervals has so far shown variable results in controlling shoot blight. When mixing Messenger, do not use chlorinated water, or water below pH 5.0 or above pH 10.0. Follow label instructions regarding tank mixing. Research on the use of this new material is continuing.

Blight Ban: BlightBan A506 is a biological control agent that is used primarily on the west coast for the management of blossom blight. BlightBan contains beneficial bacteria that, when applied to the blossoms, colonize the blossoms quickly to produce a protective barrier that inhibits infection from the fire blight bacterium. It has given significant control of blossom blight in research trials, but is usually not as effective as streptomycin. Unfortunately, BlightBan is not registered for use in New York.

How do these chemicals fit in to a blossom blight management program?

Effectively managing fire blight requires a combination of disease management practices. Chemical control will be less effective in orchards where fire blight cankers have not been pruned out. Dormant and seasonal pruning (i.e., removing infected limbs as soon as symptoms are detected and before extensive damage develops) play an integral role in reducing the amount of disease pressure in an orchard. Copper applied at 1/4-inch green will reduce the amount of inoculum on the surface of infected trees.

Assuming that appropriate prebloom controls have been employed (removal of cankers, copper at green tip), how does one incorporate Messenger and streptomycin into a blossom blight spray schedule? At bloom, streptomycin sprays are still the most effective defense against blossom blight. These sprays are critical because effective early season disease control often prevents the disease from becoming established in an orchard. Predictive models, particularly MARYBLYT, help to identify potential infection periods and improve the timing of antibiotic treatments.

Messenger can be applied 5—7 days prior to an infection event to activate a plant's natural defenses to fire blight. Accurately predicting an infection event a week in advance is impossible. Therefore, it is recommended that Messenger be applied at the pink bud stage in orchards that have historically high disease pressure. These orchards should then receive the normal streptomycin sprays in addition to the Messenger spray at pink. Thus, until we have more experience with Messenger, it should be used only as a means of adding extra protection to existing streptomycin programs. Messenger should not be used as a substitute for streptomycin.

Managing fire blight after bloom

Management focuses on minimizing shoot blight (especially if blossom blight was severe) and the development of cankers that can overwinter and serve as next year's inoculum source. Minimizing shoot blight damage begins by pruning out infected limbs as soon as symptoms are detected and before extensive necrosis develops. When pruning out active cankers, cuts should be made at least 12 inches below symptoms. The effectiveness of sterilizing pruning-shears between cuts is debatable, and is often not done due to the impracticality.

Growers should use management systems that promote early cessation of tree growth without adversely affecting tree vigor. Excessive vigor is an important component of orchard risk for fire blight. When tree growth continues past mid summer, the likelihood that late season or trauma blight infections will overwinter increases. Nitrogen fertilizer should be applied based on foliar analysis. In young blocks, it is possible to use Apogee as a means to terminate growth and possibly minimize the damage due to fire blight. The thoughts behind this approach will be featured in an article next week.

Trauma events (hail, high winds) can put any orchard block at risk because varieties that are considered relatively resistant to blossom blight and shoot blight can suffer severe blight under trauma conditions. If a trauma event occurs when trees are actively growing, application of streptomycin within 12—24 hours after the trauma event may limit the severity of the resultant trauma blight. After midsummer, when trees have hardened off for the season, streptomycin protection following trauma events may be unnecessary because trees are thought to be fairly resistant to fire blight after tree growth stops for the season. Applications of streptomycin may be not be possible after mid-summer anyway because of the days-to-harvest limitations on the label.

Managing fire blight requires season-long attention. Unfortunately, even the most prudent grower can suffer extensive losses because fire blight can strike very rapidly and unexpectedly. Next week's article will discuss the use of Apogee for managing fire blight.