Management of apple scab in orchards with existing scab lesions

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.  

2009 has already gotten off to a horrendous start regarding apple scab, first with the realization that resistance to strobilurin fungicides is present in Michigan and second with the extensive amount of rain and number of scab infection periods. Scab lesions are showing up now where the fungus was not effectively controlled during infection periods that occurred in late April. Scab lesions are sources of large numbers of secondary spores (conidia) that continue to disseminate the fungus throughout the summer. All orchard blocks should be scouted on a regular basis this spring to determine if scab lesions are present.

What are some strategies to limit the amount of infection continuing in the season and, most importantly, to limit fruit infection?

Protection, protection, protection. It is critical right now to maintain thorough coverage of tissue. With the existing fungicide resistance problems in Michigan, the best choice right now is probably the broad-spectrum protectants, EBDCs and Captan. These fungicides should be used at full rates on a five to seven day schedule with all middles sprayed to ensure proper coverage. Fungicides should be applied in advance of rains. Keep the intervals tight. This protectant strategy should be followed at least until after second cover to minimize infection of newly emerging terminal leaves and developing fruit. Any strategy (i.e. lower rates, spraying alternate middles) that does not ensure full coverage will result in either unprotected tissue or tissue with fungicide at rates ineffective for control. Since the scab spore population will be higher in orchards with existing lesions, the population will likely find unprotected tissue and infect it in these situations.

Attempts to burn out existing lesions with dodine (Syllit) are another possibility. Syllit should be used at a high rate and also tank mixed with an EBDC or Captan. Resistance to dodine is widespread in Michigan orchards and is persistent over time although the percentage of resistant fungal isolates declines over time. Thus, if Syllit has not been used in the orchard for a period of seven to eight or more years, the percentage of resistant isolates in the orchard should be low enough that the Syllit will be effective. However, once Syllit is applied, the percentage of resistance will likely increase again.

A post-infection strategy using sterol inhibitor fungicides can also be tried. Again, resistance to sterol inhibitors is common in Michigan, but appears to differ among regions. Also, sterol inhibitor resistance is quantitative, meaning increased doses of sterol inhibotor fungicide or use of sterol inhibitor fungicides with increased activity can still be effective. The probable effectiveness of an sterol inhibitors are almost impossible to predict because the percentage of resistant strains and their levels of resistance is distinct in every orchard. If a sterol inhibitor is chosen, the best alternatives available are Inspire Super MP and Indar 2F. Inspire Super (containing difenoconazole) and Indar both control a larger spectrum of scab isolates and than other sterol inhibitor’s and can potentially control some sterol inhibitor-resistant isolates. Thus, if an orchard harbors scab populations that are shifting towards resistance or include some resistant isolates, Inspire Super and Indar should be effective. In any case, sterol inhibitors should be tank mixed with a protectant and the intervals and coverage strategies discussed above apply.

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