MSU Extension

With the warm (I would say hot) weather this past weekend, orchards in Southwest Michigan should all be at or approaching green tip and other regions will not be too far behind. Although we are well behind last year in terms of development, the party is over and apple scab season 2011 is ready to fire up. During this spring and even under snowcover, the apple scab fungus has been undergoing its sexual cycle and producing primary ascospores in infected leaves that have overwintered on the orchard floor. Spore release is triggered by wetting events and occurs mostly during daylight hours. The number of hours of wetting it takes for an infection event to occur (spore release, landing on green tissue, spore germination, growth, and infection) decreases as temperatures warm, to a minimum of 9 hours when temperatures average between 61 and 75ºF (see page 84 of the 2011 Michigan Fruit Management Guide for the infection period table). It is always good to review the table as a reminder that more extended wetting periods at cooler and cold temperatures still qualify as apple scab infection periods.

If you had scab in your orchard last year, hopefully you have made some effort at reducing overwintering inoculum via urea application to fallen leaves or leaf shredding by flail mowing (see a previous article from September 21, 2010). This is a proven practice in inoculum reduction that has become more and more important each year due to our issues with fungicide resistance.

As we were in 2010, we continue to be limited in the availability of fungicide modes of action for apple scab control. Accompanying this fact is the realization that scab is much more difficult to control if inoculum levels are high compared to low. One factor that can strongly influence the amount of scab inoculum in an orchard is early infection. If primary scab infections occur early, at green tip for example, those lesions will be producing secondary spores (conidia) at a timing between pink and petal fall that coincides with what is typically the period of highest primary spore concentration. Thus, early infections can be a killer because they compound the spore load in an orchard, which can lead to significant fruit infection. Yes, there is not a lot of green tissue present in orchards at green tip. But, be certain that spores of the scab fungus can find that tissue; for any spores released at this timing, that is their primary function. And once they land on that susceptible tissue, infection, producing a lesion and conidia becomes the primary function.

When spore loads are high in an orchard, excellent fungicide coverage is essential. The best tactic to use to maximize fungicide coverage is to spray all middles.

We have been intensively sampling the scab population in Michigan for the past several years. Most scab isolates are resistant to strobilurin fungicides and also are resistant to SI fungicides. Thus, both of these fungicide classes will be ineffective in scab control. If you are using these classes of fungicides to control other diseases (powdery mildew, black rot, summer diseases), you must remember they will not be effective against scab.

All fungicides used in 2011 should be used as protectants, i.e. sprayed ahead of infection periods. The “blanket” concept is critical here; the protectant spray blankets the orchard providing a barrier on green tissue that kills spores that land there. Killing spores and minimizing any growth of the scab fungus is the best way to prevent the occurrence of mutations that can lead to fungicide resistance.

What fungicide choices are still effective in 2010?

EBDCs, Captan. These fungicides are referred to as “contact” or protectant fungicides as they provide a surface barrier on leaves and fruit that kills scab spores and germinating spores. These fungicides include Dithane, Penncozeb, Manzate, Polyram and Captan. These scab protectants typically provide five to six days of protectant activity when used at full rates.

Combinations of Mancozeb fungicides (Dithane, Penncozeb, Manzate) with Captan are especially effective because they combine the excellent retention properties of Mancozeb with the better redistribution properties of Captan. Remember that Captan is not compatible with oil. Redistribution is critical when we experience periods of warmer weather leading to rapid leaf expansion between spray applications.

Anilinopyrimidines – include Vangard and Scala. These are effective scab materials, but at risk for resistance development. At a minimum, these fungicides should be tank-mixed with a 3 lbs/acre rate of EBDC for resistance management. This class of fungicide is more effective in colder weather. This is a highly systemic material that doesn’t redistribute well and is not as effective in controlling scab on fruit. It is a good choice for early-season scab control.

Copper.  This is a good green tip spray material for scab control. Copper, at 2 lbs. metallic equivalent per acre represents a separate mode of action that is not at risk of resistance development. Also, copper at this timing will provide some control of fire blight, killing bacteria as they emerge from cankers, unless we receive over 3 inches of rain following the application.

Sterol Inhibitors. Resistance to sterol inhibitors (SI’s) in the scab fungus is widely distributed in Michigan orchards. This resistance affects the so-called 1^st-generations SI’s including Rally, Rubigan, Procure, and Topguard. There are 2^nd-generation SI’s available including Inspire Super and Indar. The 2^nd-generation SI’s are more effective against scab than the 1^st-generation SI’s, and also can control scab in orchards with known SI resistance. However, I would caution against reliance on these materials, particularly with SI resistance at very high levels in Michigan. At a minimum, either Inspire Super or Indar should be tank-mixed with an EBDC to help control SI-resistant scab strains.

Also remember that Inspire Super is actually a combination of two fungicides (the 2^nd-generation SI difenoconazole and Vangard). Thus, there are two modes of action in there, but the Vangard component is more effective under cooler conditions. An EBDC should still be tank-mixed with Inspire Super because both components of Inspire Super are at risk of resistance development.

Finally, remember that continued use of 2^nd-generation SI fungicides is predicted to increase the overall level of SI resistance in orchards.

Sulfur, Ziram. These are weaker protectants. Their shorter duration of protectant activity means more applications are required.

Increasing problems with fungicide resistance in Michigan are requiring alterations in the management of apple scab for 2011 and beyond. I anticipate that management programs will be costly, tiring, and perhaps exasperating. I remember that this was the case last year. Unfortunately, it will be the case for the foreseeable future. We need to realize that the scab fungus does not get “tired.” The nature of this fungus is to produce large quantities of spores that can be wind and rain-dispersed to green tissue and fruit of apples. With high inoculum, multiply those numbers many times over. This is why complete and consistent fungicide coverage is essential. Because of the high numbers of spores, there will always be another spore during the primary scab season that can find unprotected tissue. Keep that tissue protected!