Scaffolds 01 index

Decision #4: Configuring SI and strobilurin sprays

There are no clear-cut rules concerning the best ways to configure SI and strobilurin sprays. Using an SI+mancozeb combination at tight cluster (TC) and pink followed by a strobilurin at petal fall (PF) and 1st cover is generally no better or worse than using the fungicides in the reverse order. Depending on the situation, the following factors might affect the decision:

• The SI+mancozeb combination will be weak against black rot infections that might occur at PF and 1st cover. Substituting captan for mancozeb in the SI combination at PF and 1st cover would beef up the black rot activity of the SI+contact combination, but it would reduce activity against primary infections of flyspeck. Strobilurins used at PF and 1st cover will provide excellent protection against both black rot and flyspeck.

• The SI fungicides are slightly stronger than the strobilurins against powdery mildew, although the differences are minor when mildewcide programs are initiated before bloom. If no mildewcides are applied before PF, then an SI should always be used at PF because SI's provide a faster "knock-down" of a running mildew epidemic than do strobilurins. (Where no mildewcides are applied to mildew-susceptible cultivars before PF, mildew is almost always in the epidemic phase at PF, even though most growers fail to recognize the epidemic until about 1st cover!) If an SI+contact is needed at PF to stem a mildew epidemic, the PF spray can still be followed by a strobilurin at 1st cover to pick up black rot that might otherwise invade the retained fruitlets killed by thinning sprays.

• The strobilurins will not provide adequate control of rust diseases. Quince rust and cedar apple rust infections on fruit usually occur between TC and the end of bloom, so one might argue that SI's should be used at TC and pink in areas where rust is a concern. However, most cedar apple rust infections on leaves occur during the terminal shoot growth flush that occurs between PF and 2nd cover. Rust-susceptible cultivars sprayed with strobilurins at PF and 1st cover can develop enough leaf rust to give the tree an unhealthy appearance, even though the leaf infection may not have much economic impact on the crop.

• Currently, we can suggest four options where rust is a problem:

1. Stay with SI+mancozeb combinations from TC through 1st cover. I don't like this option because there is too much selection pressure for development of SI-resistant powdery mildew when SI's are the only mildewcides used.

2. Combine 2—3 lb/A of mancozeb with strobilurins anytime the strobilurins are used between TC and 1st cover. The low rate of mancozeb may provide more complete protection at TC and pink than at PF and 1st cover because the rapid development of terminal leaves after PF may allow the tree to outgrow the mancozeb coverage. I have not tested strobilurin+mancozeb combinations, but I cannot foresee any problems with this approach, although it adds to costs.

3. Apply a strobilurin at TC, SI+mancozeb at pink, strobilurin at PF, and SI+mancozeb at 1st cover. The SI fungicides have such potent post-infection and pre-symptom activity against rust that this alternating schedule is likely to provide adequate rust control. (I say 'likely', because this strategy has not been extensively tested.) A potential weak point in this strategy is that a severe quince rust infection occurring right at tight cluster might not be fully controlled by an SI applied 10 days later. However, if one begins with the SI at tight cluster, then infections at 1st cover would not be controlled unless another SI was applied at 2nd cover.

4. An untested alternative to a strobilurin+mancozeb combination would be a strobilurin+Bayleton combination. Bayleton will control rust as effectively as the other SI's and it would enhance the mildew control provided by strobilurins. However, this combination may be prohibitively expensive.

• Should strobilurins be used in combination with a contact fungicide? An obvious reason for using strobilurin+contact combinations is to gain better control of rust diseases. A second reason might be to compensate for poor spray coverage: presumably the contact fungicide will redistribute better than the strobilurins. A third reason for using combinations is to reduce selection pressure for strobilurin-resistant strains of apple scab. However, the latter is presumably of minor importance so long as strobilurins are used according to label instructions (no more than three sprays back-to-back).

• There is absolutely no justification for cutting strobilurin rates in half when adding mancozeb or captan to the mix. Cutting rates with strobilurins will significantly reduce the protectant, post-infection, and anti-sporulant activity of the strobilurin component and may speed selection of strobilurin-resistant strains. Thus, using strobilurins in combination with a contact fungicide will be significantly more expensive than either using strobilurins alone or than using SI+contact combinations.

Decision #5: Timing sprays during bloom

Spray timing with SI and strobilurin fungicides is often mentioned as coinciding with TC, pink, PF, and 1st cover. In fact, those phenological stages rarely occur at precisely 10-day intervals, the standard suggested spray interval for these fungicides. The biggest divergence often occurs between pink and petal fall. Growers who apply a spray at pink or early bloom often find that 10 days later they are still about 5—6 days away from petal fall. Rather than squeezing another SI or strobilurin spray in between pink and PF, it often makes sense to revert to a contact fungicide (used alone) to bridge the gap between the pink and PF sprays.

When using this strategy, the contact fungicide application during bloom should be made ahead of infection periods that are predicted anytime after the 6—7 days of protection provided by the pink spray. Remember that with both SI's and strobilurins, the 10-day interval between sprays derives from approximately 6 days of protection and four days of post-infection activity. A contact fungicide applied during bloom will not have post-infection activity, so it must be applied ahead of infection periods during mid-bloom.

Decision #6: Petal fall and first cover

In my opinion, every apple grower should be using either a strobilurin or an SI+contact combination at PF and 1st cover. This is the peak risk period for powdery mildew and for fruit infection by apple scab. Using the best fungicides during this window is just common sense. If no mildewcides were applied before petal fall, then an SI+contact combination is preferred at PF (see the discussion under Decision #4 above).

Decision #7: Second cover to mid-July

Assuming that scab was controlled earlier, the main concern during this interval is mildew. On mildew-susceptible cultivars, mildewcide coverage should be maintained until terminal shoots stop growing. The 2nd cover spray is especially important since many trees will still be in the midst of their growth flush at second cover. Where Benlate or Topsin M still work against mildew, they can be used here instead of the SI's or strobilurins. If scab was not completely controlled by earlier sprays, then a strobilurin applied at 2nd cover will more than pay for itself. The strobilurins have excellent retention on fruit, especially after the fruit begins to develop a visible cuticle, because these fungicides are chemically bound to waxes in the cuticle.