Use fungicides for apple scab protection prior to anticipated early infection periods

Growers should be protected against the apple scab pathogen prior to this weekend’s rain event.

The sustained heat in Michigan right now is unprecedented and has caught many by surprise in terms of how rapidly apples are developing. With rains predicted for Friday, March 23, and for next week, it is critical that scab-susceptible cultivars are protected from this anticipated early scab infection event.

In the early season of 2010 (which still wasn’t as early as we are now), the scab fungus was a little behind apple trees in development because we had an extended period of snow cover leading into the season that kept the fungus present in overwintering leaves cold. The lack of snow cover this “spring” of 2012 and lack of real cold temperatures during the winter suggests that the scab fungus is ready to go now.

Apples in southwest Michigan are at tight cluster and at 0.5-inch green tip on the Ridge. Protecting trees during this period of incredibly rapid development is an essential first step to preventing an apple scab epidemic. We will likely have a high spore load out there for this upcoming first scab infection period. Although there is not a huge amount of green tissue present, scab infections at green tip can have severe economic consequences. This is because scab lesions that are initiated around green tip will usually produce conidia between pink and petal fall, the traditional timing when primary ascospores are in highest numbers. It will be extremely difficult to control scab under such high inoculum pressure and with the tree growth at the later timings where rapid growth results in more unprotected tissue between fungicide applications.

The best fungicides available for scab control at this time of the early season are the broad-spectrum protectants: Captan and the EBDCs. It is likely too late for copper (see previous article, “An early-season copper application will help avoid feeling ‘the blues’ about diseases”). Also, it is too hot for the anilinopyrimidines (Scala and Vangard) that have better efficacy at cooler temperatures (highs in the low 60s and below). A tank-mix of Captan (3 lbs/A Captan 50W) and EBDC (3 lbs) is an excellent scab control combination. This combination takes advantage of the efficacy of both materials and the superior retention and redistribution of the EBDCs. Spray intervals need to be tighter than normal because of the amount of new growth. Also, be careful with Captan, as use of Captan with oils or some foliar fertilizers can lead to phytotoxicity.

We are hearing a lot of concern (fully warranted) about the prospect of a crop for 2012. We can’t predict the weather, but controlling scab early is critical. If we let scab take hold early, and we do have a crop, the fungus will get the crop later. Scab is one factor we can control in this early season – let’s do it!

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

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