Apple Disease Update: Week of June 5, 2022

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Early symptoms of Bitter rot on fruit

Last week’s post could not have been more timely concerning Glomerella leaf spot and bitter rot. Unfortunately the disease was on its typical Memorial Day weekend schedule, and we are seeing symptoms in orchards in both Henderson and Polk County. Two observations so far in the few orchards in which I’ve seen it:

  1. There was an incredible amount of leaf litter still remaining under the trees from the previous season. This leaf litter serves as a reservoir for the overwintering pathogen and spores are being released during rainfalls.
  2. Fungicide intervals have been well extended beyond the recommended 7-10 days.

‘Gala’ and ‘Golden Delicious’ have been the two cultivars so far in which we’ve seen symptoms. Symptoms were more advanced than I would have anticipated, but the good news is that I’m not seeing too many new leaf lesions emerging. Scrolling back through the weather for Mills River on the NEWA website, the last 10 days of May and first 2 days of June the precipitation and/or leaf wetting hours combined with the day and nighttime temps aligned perfectly for GLS/bitter rot issues.

I would advise you to go out and scout cultivars that are highly susceptible to GLS or bitter rot and identify any hot spots. Also, if you’ve got a knack for detecting GLS try to look for early infections:Early GLS on leaves

As I mentioned, some infections that we’ve been observing are relatively advanced:

Advanced GLS Symptoms on Golden Delicious

If you missed last week’s post, here my most important take home messages regarding GLS management:

  • DO NOT extend fungicide application intervals from now through harvest on ANY cultivar that is susceptible to GLS OR any cultivar that you consistently observe a lot of bitter rot (e.g. Honeycrisp). The old “14-21” spray interval guidance should be thrown out the window with these two diseases and with the highly susceptible cultivars we are growing in the region.
  • That said, 7 to 10 day interval is the sweet spot. If its Honeycrisp or a block that has heavy GLS or bitter rot pressure in the past, err on the 7 day side. If we get a lot of precipitation, you may want to shorten this up even further on cultivars like HC. Studies have shown that it’s actually a single precipitation event that gets rid of most of the captan deposit from a single spray. The product tends to remain after that.
  • The most efficacious fungicides for managing GLS and bitter rot have been Mancozeb, captan, ziram (if surfactant is included according to research being conducted at PSU), and FRAC 11 fungicides: Pristine, Merivon, Flint Extra, Luna Sensation, the former two having better kickback activity. For those unfamiliar with kickback activity, this simply means that the product works after a spore lands and germinates. The strobilurins are “ok” in general in regards to kickback activity but I would not make a habit of this, nor would I go past 24 hours after an infection event.
  • Captan alone (half rate) or Captan (half rate) plus ProPhyt (3 pt) or Captan (half rate) + Ziram 3 lb should be the backbone of your summer covers. Perhaps rotate between those three options. If you’re concerned about too much captan and hitting your annual limit, Ziram 4 lbs + surfactant or a strobilurin (listed above) plus Ziram 3 lbs are options.

You can read my other tips on this Apple Disease Update.

The other disease warning I’d like to give this week is on Marssonina Leaf Blotch. Next week my post will focus more extensively on Marssonina, but for the purpose this week I simply want you to be aware of early symptoms. In general, early symptoms of MLB on leaves look quite similar to early symptoms of GLS and frogeye leaf spot- purple to red spots or flecks on leaves. In general we’ve found that early MLB spots tend to be more clustered together on one side of a leaf but this always isn’t the case. My recommendation is to head out to Rome Beauty or Fuji, two cultivars where we have already identified symptoms this year, and check for any symptoms. This won’t help you differentiate between frogeye and MLB, but GLS does not occur (to our knowledge) on these cultivars.

Early MLB non leaves

For now, stay on course with the protectant fungicide plan for both diseases. If you haven’t applied a stroby yet this season or have just applied one application of a stroby (FRAC 11) such as Merivon or Pristine or Luna Sensation or Flint, I would consider applying one of these fungicides with either a half rate of Ziram or Captan this week.