Scaffolds 01 index

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

Spore maturity in the Hudson Valley is unusually advanced compared to tree phenology. Trees still had no green tissue present at the time of this spore count. Significant spore discharges usually begin when we reach 15% mature spores in our counts. Although very few spores were discharged in the tower shoot test, that could change quickly with a few days of warmer weather. (Tower discharges usually exceed 400 spores at the peak of the season.)

The early spore maturation is hardly surprising given the constant snow cover during winter and the wet weather of the past two weeks. The early spore maturation in the Hudson Valley is similar to that of 1998 when many orchards developed severe scab as a result of a green-tip infection period followed by a wet spring and summer. Thus, Hudson Valley growers should be prepared to spray as soon as there is green tissue and a predicted infection period, even if they can only spray alternate rows. With high carry-over inoculum present in many orchards, this will not be the year to take chances on skipping early infection periods.

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

The last two issues of Scaffolds Fruit Journal contained articles discussing scab and mildew control strategies for orchards where SI fungicides (Rubigan, Nova, Procure) are still effective for controlling apple scab and powdery mildew. Strategies suggested in those articles must be modified for orchards where the SI fungicides no longer provide acceptable control of apple scab or mildew.

Status of fungicide resistance in apple orchards: Apple scab has developed resistance to dodine (Syllit), the benzimidazoles (Benlate, Topsin M), and the SI fungicides. Apple scab has never developed resistance to any of the "contact" fungicides, a broad grouping that includes the EBDC fungicides (mancozeb, Polyram), captan, the other carbamates (ferbam, thiram, ziram), or sulfur. The contact fungicides are multi-site inhibitors. That means that they disrupt several different metabolic pathways in fungi, thereby making it difficult for the fungus to circumvent the action of the fungicide. By comparison, the fungicides to which apple scab has developed resistance are single-site inhibitors. They arrest fungal development by interfering with a single critical metabolic pathway in the fungus. Resistant strains of apple scab survive the fungicide either by using an alternative metabolic pathway or by limiting access of the fungicide to the critical pathways within the cells.

Fungicide resistance comes in two flavors. With benzimidazole fungicides, resistance is an "all or nothing" proposition. That means that strains of the apple scab fungus that are resistant to the benzimidazoles cannot be controlled by using higher rates of Benlate or Topsin M. Resistant strains will grow through any rate of fungicide that could be applied in the field.

Resistance to dodine and the SI fungicides develops in a gradual "step-wise" process. Wild populations of apple scab contain strains with varying levels of sensitivity to dodine and SI fungicides. Repeated use of these fungicides gradually eliminates the most susceptible strains from the population, leaving only the less sensitive strains. These less sensitive strains can still be controlled with higher rates of the fungicides, but they survive in the field when fungicide rates are low, when intervals between sprays are too long, or when spray coverage is incomplete. This gradual selection for resistance ultimately produces stable scab populations that cannot be controlled by applying fungicides at labeled rates and intervals.

Resistance to dodine and benzimidazole fungicides is so widespread in New York State that these fungicides are no longer recommended for control of apple scab. They are still effective in some orchards, but their performance is unpredictable because of the unpredictable distribution of fungicide resistance. Furthermore, Drs. Wolfram Koeller and Wayne Wilcox at the Geneva Experiment Station have shown that a few orchards also contain scab populations with levels of SI resistance sufficient to account for failure of the SI fungicides under field conditions. Growers and fieldmen have also reported that SI fungicides are no longer controlling powdery mildew as effectively as when these fungicides were first introduced. However, levels of SI resistance in powdery mildew have not been documented by researchers.

The extent of SI resistance in apple orchards is difficult to determine because testing for resistance is a tedious and expensive process. As a result, most assessments of SI resistance in commercial orchards must be based on grower experience: If the SI fungicides fail to provide good control of scab or mildew when they are applied in a conservative 10-day program (as always, in combinations with contact fungicides), then quite possibly the scab and/or mildew population in that orchard has shifted toward SI-resistance.

Adjusting programs to compensate for SI resistance: Apple growers with SI-resistant apple scab and/or powdery mildew are at a significant disadvantage compared with growers where SI fungicides are still effective. The new strobilurin fungicides (Sovran, Flint) can control scab and mildew as well as SI fungicides, but the strobilurins represent the last available chemistry with post-infection activity against apple scab and good protectant activity against mildew. Where SI fungicides are no longer effective, growers should take extra precautions (= extra expense!) to protect strobilurins against resistance development. We do not know how quickly scab and mildew may develop resistance to Sovran and Flint, but we know that resistant strains will eventually appear if these products are used inappropriately.

Following are my suggestions for using Sovran and Flint in orchards where growers suspect that apple scab or mildew has become resistant to the SI fungicides:

1. Use a very conservative program of contact fungicides from green tip to tight cluster or pink. Resistance to Sovran and Flint can be expected in orchards where these strobilurin fungicides are routinely used as "clean-up" materials to compensate for missed infection periods earlier in the season.

2. Where SI-resistant scab is present, Sovran and Flint should be used in combinations with a contact fungicide (either mancozeb, Polyram, or Captan). Although Sovran and Flint have been tested as "stand-alone" fungicides, tank mixes are essential for orchards where alternating with SI fungicides is no longer an effective resistance management option. As indicated in last week's article, rates of Sovran and Flint should never be reduced below the standard dilute rates (equivalent to 0.67 oz of Flint or 1.33 oz of Sovran per 100 gallons), and this applies even when the products are used in combinations. Tree-row volume adjustments are acceptable down to the dilute equivalent of 100 gal/A. Thus, even on very small trees, the minimum rate per acre should never be less than 0.67 oz of Flint or 1.33 oz of Sovran.

3. Shift the use Sovran + contact fungicide or Flint + contact fungicide forward in the growing season compared to what might otherwise be considered optimum timing. Thus, where SI resistance is already present, the strobilurin + contact combinations should probably be applied at pink and petal fall (or at pink, petal fall, and first cover) rather than just at petal fall and first cover. In very wet years with high carry-over inoculum, the first strobilurin spray might be justified at tight cluster. The objective here is to eliminate any potential for letting scab or mildew infections become established before the strobilurin fungicides are applied. The peak period of risk for primary scab infections usually occurs near pink, so it makes sense to target the strongest fungicides toward this time period. (As mentioned last week, a mancozeb or captan spray may be needed during bloom to bridge the gap from pink to petal fall.) This forward shifting of the strobilurin sprays is especially critical for orchards where growers believe that SI fungicides are no longer controlling mildew because mildew is usually well established by petal fall and the strobilurins act more slowly than the SI fungicides for arresting a running population of mildew.

4. The strobilurin + contact fungicide combination can be used only three times in direct succession. Where mildew is a significant problem, additional sprays will be needed after petal fall or first cover. The best option here (or where scab is still a threat after first cover) is to follow up with an SI + contact spray. If activity of SI's against mildew has been declining, then higher rates and tighter spray intervals may be needed. In some young orchards, Benlate or Topsin M may provide effective control of mildew in cover sprays, but benzimidazole-resistant isolates are present on many older orchards. The only other alternative for mildew is to include sulfur in the early cover sprays.

These guidelines using Sovran and Flint in orchards with SI resistance are based on our current understanding of fungicide activity and resistance management. The suggested guidelines may change as we gain a better understanding of resistance development in strobilurin fungicides and/or the extent of SI resistance in apple orchards.