MSU Extension

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

We are in the thick of apple scab weather right now. As your dedicated state Extension personnel (Mira Danilovich, Amy Irish-Brown, Erin Lizotte, Mark Longstroth, Nikki Rothwell, Phil Schwallier, Bill Shane, Bob Tritten, Diane Brown-Rytlewski) have been telling you, we already have experienced scab infection periods during the first week of April. While orchard spore counts have been relatively low, we need to remember a few things:

We’re in an early season with tree development two to three weeks ahead of normal. It is quite possible that the scab fungus is lagging just slightly this spring in development and spore production. However, last year was a high scab year in most regions. Thus, most orchards would be high-inoculum orchards coming into 2010. Furthermore, since most scab isolates from last year were resistant to strobilurin and sterol inhibitor fungicides, we have only a few fungicide options for 2010.

We must remember that it is much more difficult to effectively control apple scab in a high-inoculum orchard versus a low-inoculum orchard.

Since McIntosh is so highly susceptible to apple scab, controlling scab effectively in a high-inoculum McIntosh orchard will be supremely difficult in 2010. Keep especially vigilant with scab control on all McIntosh blocks.This means that thorough fungicide coverage is absolutely critical throughout the primary scab period through post-bloom into first cover.

I think that we will be seeing plenty of scab spores released with rain events over the next few weeks. With the lack of post-infection tools available, keeping trees covered in advance of rains is critical.

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, 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. Highly systemic material that doesn’t redistribute well and is not as effective in controlling scab on fruit. Good choice for early-season scab control. We are rapidly approaching the end of the best window for use of this class of scab fungicides.

Sterol Inhibitors – resistance to sterol inhibitors (SI’s) in the scab fungus is widely distributed in Michigan orchards. This resistance affects the so-called first generation SI’s including Rally, Rubigan and Procure. There are second generation SI’s available including Inspire Super and Indar. The second generation SI’s are more effective against scab than the first 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 second 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 second generation SI fungicides is predicted to increase the overall level of SI resistance in orchards.

Sulfur, Ziram – weaker protectants, shorter duration of protectant activity means more applications required.

Increasing problems with fungicide resistance in Michigan are requiring alterations in the management of this disease for 2010 and beyond. I anticipate that management programs will be costly, tiring, and perhaps exasperating. Unfortunately, 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.

Dr. Sundin's work is funded in part by MSU's AgBioResearch.