Management of apple scab in orchards with existing scab lesions
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EmailEditor’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.
