1996-1997 New England Apple Pest Management Guide


 SCAB MANAGEMENT STRATEGIES

1996-1997 New England Apple Pest Management Guide, pp. 12-15

1) PREVENTION. Scab-resistant cultivars offer the possibility of not having to use fungicide to control scab, but chemical control is essential in orchards planted with cultivars rated moderately or highly susceptible to scab (see Table 3). The best preventive measure is successful use of fungicide to control scab the previous season so that there will be a low level of ascosporic inoculum in spring.

Cultural practices. Cultural practices to reduce inoculum and enhance the efficacy of fungicides are also useful in scab management, especially in an orchard that was not well managed for scab last year.

Pruning to open the tree canopy promotes penetration by air and light to reduce the time it takes for wet leaf and fruit surfaces to dry, and this will also reduce the number of lesions that would develop if the trees remained wet longer. Opening the tree canopy also allows greater penetration of pesticide spray.

Removing unsprayed susceptible trees (e.g., flowering crabs and abandoned apple trees) from within 100 yards of an orchard will reduce the number of ascospores entering the orchard from outside sources to a level that will not contribute significantly to scab development in the orchard.

Sanitation practices. Research has demonstrated that sanitation practices such as flail&shyp;mowing fallen leaves in autumn or early spring (before bud break) or applying urea to fallen leaves decreases the amount of ascosporic inoculum approximately 50-75%. This means that for any one infection period, there would be approximately 50-75% fewer scab lesions compared to the number of lesions that would develop if there had been no sanitation practice.

When should sanitation practices be used? Sanitation practices will not be cost effective unless they reduce scab management costs during the growing season. Whether or not the sanitation program presented below is cost effective will depend on each grower's situation.

Sanitation program
Assess the orchard for foliar scab (see Scab Management Strategies: Potential Ascospore Dose Assessment). If the number of foliar scab lesions falls within the sanitation action threshold (see next section), select the sanitation practices that best fit your situation


2) DELAYED SPRAY STRATEGY
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This is a new strategy recommendation, and it is based on 10 years of research in grower's orchards in New Hampshire, Vermont, Maine, and Massachusetts. You will not be able to delay sprays according to this strategy until performing the postharvest foliar scab procedure described below. Because this strategy is new, you should try it in only one block the first year; preferably a block of small to moderately tall trees on semidwarfing rootstock in a newer planting that has had good groundcover management and has been wellmanaged for scab. Old blocks with large trees are not acceptable for this strategy.


Predicting Next Year's Scab Potential

To use the DelayedSpray Strategy, you must assess foliar scab in the preceding autumn before leaffall, using the following technique:

a. 20 shoots on each of 30 trees (for a block of 300-900 trees).
b. 15 shoots on each of 40 trees (for a block of 400-1200 trees).
c. 10 shoots on each of 60 trees (for a block of 600-1800 trees).


If the number of scabbed leaves is 50 or less, the first fungicide spray can be delayed according to the following action threshold:

DELAY THE FIRST FUNGICIDE SPRAY UNTIL PINK OR UNTIL AFTER THREE INFECTION PERIODS (but before the 4TH Infection Period), WHICHEVER COMES FIRST*
*Begin determining infection periods at green tip.

If the number of scabbed leaves is within the sanitation action threshold (50-100):
SELECT A SANITATION PROGRAM (see previous page), PERFORM THE SANITATION PRACTICES YOU SELECT, THEN DELAY THE FIRST FUNGICIDE SPRAY ACCORDING TO THE ACTION THRESHOLD.

If the number of scabbed leaves is greater than 100: DO NOT SELECT THE DELAYED SPRAY STRATEGY.


3) PROTECTION STRATEGY. This strategy is designed to maintain a protective fungicide residue barrier on, or in, leaves and fruits. A protective fungicide schedule historically has been recommended to begin at green tip, with subsequent treatments timed according to fruit bud stages or a 7 day calendar schedule during the primary scab season. Research has shown that even with over two inches of rain, the 7 day schedule is sufficient if captan, dodine, or an EBDC fungicide is used.

If strictly followed, the 7 day schedule may result in fungicide being applied well before it was needed. When seven days have passed since the previous application and no rain is expected, the next protective application can be delayed until just before the next rain is forecast. But the unpredictability of rainfall poses the risk of an unexpected infection period occurring when the trees are not protected. The protective strategy usually requires more fungicide, labor, and equipmenttime than a postinfection strategy, but it is also an effective lowrisk strategy that does not require monitoring scab infection periods. It is the best option for growers who cannot use a post-infection strategy effectively because of labor, equipment limitations, or orchard size that don't allow enough time to apply fungicide within the "hours after infection" specified on the label.


4) POST-INFECTION STRATEGY (without sterol inhibitor fungicide). This strategy is designed to apply fungicide when there has been an infection period and the trees are not adequately protected with fungicide residue remaining from the last fungicide application. Weather equipment that monitors temperature and hours of surface wetness must be used to determine if conditions were favorable for infection. When an infection period is determined to have occurred, a fungicide with postinfection activity is applied if no fungicide was applied within the previous seven days.

A postinfection strategy can reduce the number of spray trips and the amount of fungicide used, especially during a spring with few infection periods spaced widely apart. One limitation with this strategy is that the fungicide must be applied in less than 24 hours from the beginning of an infection period, unless a sterol inhibitor (SI) fungicide is used. Rapid response is made more difficult by the fact that postinfection sprays should not be applied at low volume (over 6X) because of the need for excellent coverage. Prolonged rainy or windy weather, orchards that are too wet for travel, breakdowns, or other unexpected occurrences can also prevent timely application of a protectant. This could result in extensive infection, so it may be desirable to have an SI available for an emergency backup.

Fungicide resistance is another factor to consider. After the SI fungicides, the next best fungicides for postinfection activity are the benzimidazoles (Benlate and TopsinM) and dodine (Syllit). Unfortunately, scab can develop resistance to these materials, so repeated use is not recommended, especially as postinfection sprays. This leaves the protectant fungicides (captan, EBDC's, and thiram), none of which is noted for good postinfection activity.

5) 4 SPRAY 'SI' SCHEDULE.
This schedule recommends four applications of an SI combined with a protectant fungicide scheduled at 7-10 day intervals beginning 1-3 weeks after green tip, which is typically at tight cluster. Success with this schedule depends on the following assumptions:

The amount of ascosporic inoculum is low. This can be determined by the procedure for assessing ascosporic inoculum (see Predicting next year's scab potential). The low prediction that qualifies an orchard for the protectant delayed spray strategy beginning at the pink fruit bud stage also qualifies the orchard for the 4 spray SI schedule.

Spray coverage is adequate and spray calibration is accurate. Effective control with an SI requires that the leaves and fruits receive adequate fungicide. Disease control failure can occur if any part of the tree is not sprayed thoroughly (particularly the upper canopy) or if there is poor absorption. (See Notes on Fungicides in Part II for guidelines on using Nova, Rubigan, and Procure)

Factors such as cultivar or weather during spray application do not increase scab risk significantly. This strategy should not be used with a highly susceptible cultivar such as Jerseymac. Also, strong wind, rapid drying, or other factors that interfere with a spray application can decrease the effectiveness of an SI, and must be avoided.

Applications of an SI must be made when conditions are good for spraying rather than basing applications on infection periods. If the interval to the next spray exceeds 10 days, the label rate of the SI should be increased to maximum for the next application.

SI Resistance Management
To retard selection for SI resistant scab strains in the population, tank-mix captan or an EBDC fungicide in Pink, Petal Fall, and First Cover sprays. The protectant will also improve protection against fruit scab and black rot.

The minimum Rubigan or Procure rate should be 3 to 4 fluid ounces. per 100 gallons dilute, with the 4 oz. rate preferred in the Tight Cluster and Petal Fall sprays. Use no less than 8 fl. ozs. per acre, even if the trees are very small. The minimum rate of Captan 50 WP should be 1 lb. per 100 gals. dilute or the equivalent rate of another captan formulation. The minimum rate of an EBDC fungicide should be the Extended Program rate per 100 gals. dilute. (Note the amount per acre restriction on prebloom EBDC if you want to use an EBDC after bloom).

If Nova is used, the minimum rate should be 1.5 ozs. per 100 gals. dilute, with a 2 oz. rate preferred for the tight cluster and petal fall sprays. It is recommended that Nova not be used at less than 4 ozs. per acre, even if the trees are very small.

Considerations for the 4 spray SI schedule
Adopting this schedule as a standard practice may hasten the buildup of fungal strains resistant to SI fungicides, though consistent combination with protectant fungicide may retard the shift toward resistant strains in the pathogen population.

The schedule may not be as cost effective as the delayed spray protectant program, which can also be used for a low inoculum orchard.

Spray coverage is more critical with SIs than with protectants, and this means that errors in sprayer calibration and spraying technique with an SI can result in greater scab buildup.

Once started, the second fungicide application must also include an SI to be effective, and a full 4 spray SI schedule should be followed to be fully effective.

Extending the interval between SI sprays to longer than 10 days, for whatever reason, may result in insufficient protection before the next application.

An SI does not kill the fungus after it penetrates the cuticle: it inhibits it. We do not know what happens to the inhibited fungus, but if the inhibition is removed in autumn after the leaf falls and dies, it is conceivable that renewed growth of the fungus from the previously inhibited infections could result in a much greater ascosporic inoculum next spring than expected.

6) ERADICATION

It is recommended that orchards be checked for scab lesions at least once a week, beginning two weeks after the first scab infection period. Examine extension shoot leaves and fruit cluster leaves, making sure to examine both leaf surfaces. Check areas of the canopy most likely to have inadequate spray coverage. With large trees, this means the upper canopy, so a thorough inspection may require climbing.

Active scab lesions are those producing conidia, and they are usually olivegreen to brown with a velvety or dusty surface. Lesions that are no longer active appear blackened or grayed and do not have a 'velvety' surface. However, the appearance of scab lesions is variable, and it is not possible to tell whether or not conidia are being produced without microscopic examination.

If an SI fungicide has been applied, inadequate dosage or application problems can result in fruit scab lesions developing, even though leaf lesions were prevented. In addition to examining foliage, check 50 fruitlets per tree on at least one tree per acre or at least 5 trees per block. There may be significant risk of secondary scab if 1% or more of the fruitlets or leaves have active scab lesions.

There are too many variable factors (e.g., cultivar, orchard history, tree size, spray coverage) to establish a threshold number of leaf or fruit lesions above which eradication treatment is advised, so each grower will have to rely on previous experience with each orchard or block to determine the appropriate control response when scab lesions are observed.

Fungicides recommended for burning out scab lesions reduce the number of conidia produced on existing lesions and reduce the ability of existing conidia to germinate. Because of the need to cover all scab lesions with fungicide, relatively high volume, low concentrate applications (no more than 4X) will give the best results.

If scab is detected during the first weeks after Petal Fall, the safest recommendation is to apply Rubigan or Nova, in combination with a protectant fungicide at full rate, through second cover to protect against the development of new lesions and suppress further development of secondary inoculum (conidia) on existing lesions. Two full rate captan applications 57 days apart is also recommended as an eradicant treatment, particularly when daytime temperatures exceed 80 F.

These treatments in summer will also help manage sooty blotch and flyspeck. Other possibilities are several applications of Benlate 50 WP or TopsinM, in combination with captan at half the full label rate, where resistance to Benlate and Topsin M has not developed and these materials have been used sparingly.

Several applications of Syllit plus a half label rate of captan is an effective scab eradication tactic where Syllit has been used sparingly and resistance to it has not developed. Syllit is not effective against summer diseases. Combining it captan will help manage summers diseases and reduce the chance of a buildup of fungal strains resistant to Syllit.

Go to Scab-Resistant Cultivars


1996-1997 New England Apple Pest Management Guide