Over the past two weeks I've spent some time visiting with apple growers throughout Henderson County, and the same concerns keep arising (and nope, it's not Glomerella or bitter rot..). Given the dry weather we've been experiencing throughout the region and the warming temperatures, powdery mildew has emerged as a challenge early this season. Certainly, the other major concern so far has been the emergence of fire blight (mostly in the shoot blight stage) throughout western NC. Let's go through game plans for each of these issues and then focus on the threat of a GLS and bitter rot infection by week's end.

The featured photo at the beginning of this post, as well as the photo immediately above is an example of signs and symptoms of primary powdery mildew infections. The powdery mildew pathogen overwinters infected buds from the previous season and when these infected buds break in the spring, malformed shoots covered in spores of the mildew fungus emerge. Often the shoots will have a bright white to silvery appearance. The infected shoots serve as the source for secondary infections as seen immediately below:

Symptoms of secondary powdery mildew infections are more limited to leaves than shoots. The photo on the left shows a powdery mildew lesion the back of the leaf. While I see secondary infections most often on the back of leaves, in severe cases lesions may be present on the top/front of the leaves. Older secondary infections present as red margins on the backside of leaves and often leaf curling is present. Fruit infections are possible under severe conditions and on highly susceptible cultivars and appear as net-like russeting.

Proper fungicide selection is important for the management of apple powdery mildew. I know several growers in this area still love to use sulfur. Sulfur is not my top choice for a few reasons:

1. It can lead to phytotoxicity under slow drying conditions and when temperatures approach 85F or above. Remember, while the air temperature may be below 85F, temperatures within the canopy or on fruit may be greater, particularly if in direct sunlight. This can lead to micro-cracks in fruit making it easier for pathogens such as Colletotrichum (causal pathogen of Glomerella fruit rot and bitter rot) to infect apple fruit.
2. It needs to be applied more frequently than modern locally systemic fungicides such as those in FRAC 3, 7, and 11.
3. It really smells, makes my eyes water, and my asthma flare up.....

I've also heard of some growers using Kaligreen or Milstop for their mildew infections. The active ingredient in these products is potassium bicarbonate and while they will help to "burn out" the mildew fungus on the surface of leaves, these products are not strong for extended protection of new infections. Also keep in mind that broad spectrum multi-site protectant fungicides such as mancozeb, captan, and ziram have little to no efficacy against apple powdery mildew.

So what are some options?

• The DMIs (FRAC 3) fungicides are generally strong against apple powdery mildew. Rhyme (Topguard) has historically performed the strongest both when I trialed it in NY and in NC. Procure is also a good FRAC 3 option. Inspire Super, Cevya and Indar 2F perform decent but I rank them slightly lower for mildew in apples
• The strobilurins (FRAC 11) such as Cabrio, Flint Extra, Sovran, and premixed products (e.g. Luna Sensation, Merivon) have also performed excellent in our 'Rome Beauty' mildew trials at the MHCREC. However, given the importance of this fungicide class for GLS and bitter rot, I wouldn't use this group for a special mildew application.
• The SDHI (FRAC 7) fungicides (e.g. Fontelis, Aprovia, Miravis) we have screened provide good but not excellent activity against powdery mildew.
• There are several powdery-mildew specific fungicides such as Torino, Vivando, and Gatten that can be rotated into summer cover fungicide applications.

I would suggest incorporating a fungicide with efficacy against powdery mildew into your cover sprays weekly for the next few weeks. Once shoots with primary mildew stop emerging, incorporating a mildew fungicide every other spray is likely sufficient for most situations. Also, remember that as leaves age, they become resistant to powdery mildew!

As I noted at the beginning, fire blight has shown up throughout Western NC. Infections can be linked back to an ugly week of predicted blossom blight infections from April 14-18. Where I've seen the most severe infections is in situations where Kudos (prohexadione-calcium was not applied). The good news is that we have been testing for streptomycin resistance, and so far none has been detected!

If you start to see symptoms of shoot blight: Blackening of the midvein of leaves (see below photo), shepherd's crook (see above photo) and wilting due to clogging of xylem vessels it's important to promptly remove the infection. If symptoms are sporadic and few, cut at least 12 to 18 inches below the leading infection margin (the fire blight pathogen will travel down the shoot towards the central leader or older wood). You may want to make spot applications with streptomycin (PPE/label rules still apply) following removal of the blighted shoots and do not prune first thing in the morning or on day when rain is predicted. Also, a recent multi-state study on shoot blight removal found that tearing out blight branches worsens shoot blight symptoms and its spread. If you shoot blight widespread throughout a block, consider an application of Kudos (12 oz/100 gal) first, and then follow up with pruning. Make sure to remove all infected pruning material from the orchard (do not just throw on the ground and mulch it in). Followup with another application of Kudos in about 14 days. You will need to go out multiple times to remove infected tissue from trees.

Given the hot temperatures at the beginning of the week, and rain predicted by Wednesday, I would aim to get a fungicide application on targeting GLS and bitter rot on Monday and/or Tuesday if you haven't made a fungicide application in the past 7 days. In regards to fungicides this week, this is the time that you'll likely be switching from mancozeb to captan for cover sprays if you haven't already done so. However, if you have not applied 21 lbs yet of Manzate Pro-Stick, Roper, Koverall, or a similar mancozeb product, AND you are not within 77 days of harvest, you may want to consider maxing out your seasonal application limit to avoid burning through your captan annual limits too quickly. Also, with the lack of availability low availability of Ziram, rationing captan is of utmost importance this year.

Some fungicide options to consider:

• Young, non-bearing trees: Since fruit rot will not be a concern, I'd consider fungicide applications every 10 days or so on GLS susceptible cultivars and every 14-21 days (depending on weather) for cultivars that are not susceptible to GLS. Powdery mildew tends to be a bigger concern in young trees as you don't want to lose any photosynthetic activity of these trees and there is often susceptible leaf tissue on young cultivars throughout much of the summer months. Captan (or mancozeb if not harvesting) is a great choice for most diseases on young trees with the exception of powdery mildew.
• It is around this time, that two applications of a FRAC 11 fungicide, such as Merivon, Cabrio, or Flint Extra should be applied on GLS susceptible cultivars. The goal being to target early infections and knock down Colletotrichum inoculum. Remember to include 2.5 lb/A of Captan 80 WDG (or Manzate 3 lb/A) with any single-site/locally systemic fungicide.
• If you've already applied two applications of a FRAC 11 fungicide or have/suspect resistance to strobilurins (FRAC 11s), include Aprovia or Omega with captan or mancozeb.