Blossom Blight and Suggested Spray Plans

As many of you know, blossom blight is prevalent this year.  It is not present on all peach varieties to the same degree, but many varieties are inundated with Monilinia fructicola (brown rot) spores on the blooms, many of which are gummed and forming cankers.  In some cases, the cankers are girdling stems and resulting in additional loss.  The total amount of inoculum present for latent infections, green fruit rot, late brown rot, and post-harvest brown rot will make it a more challenging season.  There are likely several reasons for the degree of blossom blight observed:

(1) Protracted bloom.  I do not personally remember a bloom quite like this; bloom lasted on some varieties for almost a month.  This was likely due to the fact that we started bloom, but then the temperatures were colder — especially night time temperatures.  This stop/go approach resulted in an exceptionally long infection period for many blooms.  We hit the optimum 77 F temperature often during bloom, and moisture (3-5 hours wetness) was often sufficient as well.

(2) Limited sprays on some varieties for the last two years.  The fact that many varieties were not substantively sprayed with full fungicide programs due to low chill (2017) and freezes (2018) resulted in a substantial inoculum buildup with many holdover cankers in some blocks.

(3) Frost and/or cold damage that resulted in more disease. There is limited information to support a firm connection between cold-damaged tissue and blossom blight, but it is generally accepted that it has some effect in peach and other commodities.  We did have cold damage to some degree in many varieties this year.

To encourage you, in discussions with other specialists, we agree that blossom blight cankers do not as often girdle the stems — as opposed to Phomopsis constriction canker.  In addition, no specialist has indicated that blossom blight alone will directly reduce peach yields substantially; even in the worst years, it generally only thins trees.  However, if you combine blossom blight with cold damage, thinning is beyond the desired measure.  Even with this much blossom blight, if it turns dry for much of the season, especially late, we will likely not have major brown rot pressure; infection of fruit is directly tied to moisture.  With that said, we need to make sure that we have a good action plan for the remainder of the season, and I don’t want us to expect dry weather; the current forecast is for a wet spring.

Therefore, I suggest the following fungicides be applied as a prescriptive approach to management for the remainder of this season:

(1)  Abound (15.5 fl. oz./A rate; high rate) + Captan (low rate from product use rates for peach; see individual labels) in the next two cover spray applications. The Abound should provide some antisporulent activity, and it is active on both scab and brown rot; it is also systemic to some degree. The Captan will add to the brown rot and scab control by placing a contact material on the surface, while the Abound penetrates.  Captan will also provide resistance management for the Abound.

(2) Depending on where you are in the season for a particular variety, add one more Captan application (high rate from product use rates for peach; see individual labels) as an early cover spray.  This should get us close to pit hardening on most varieties, and these first three applications should prevent latent infections.

(3) If dry, you can use sulfur in the mid-season sprays.  If there is significant rainfall and/or green fruit rot is observed, stick with Captan.  Sulfur does not control brown rot; it is only active on scab, and it is not as active as Captan on scab.

(4) At ~28 days from harvest use a high rate of Captan.

(5) At 21 days prior to harvest, use Merivon (6.7 fl. oz./A rate; high rate).

(6) At 14 days prior to harvest, use Merivon (6.7 fl. oz./A rate; high rate).

(7) At 7 days prior to harvest, use tebuconazole (high rate of whatever product is readily available; Orius is one example) or Indar (12 fl. oz./A rate; high rate as per 24C label for Georgia).

(8) If needed during the active harvest (between pickings and 7 days after the last fungicide application of tebuconazole or Indar), apply another application of either tebuconazole or Indar.  Observe the reentry intervals and PHI. 

(9) Apply Scholar in the packing line.  

Control damaging insects as much as possible, as insect damage will increase green fruit rot and late-season brown rot as well.

This is an aggressive program, but it will take into account resistance management, while also providing optimum efficacy for a year like this.  Godspeed to each of you, and I hope it will be a profitable year yet.

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About pbrannen

Phil Brannen is a Professor in the Plant Pathology Department at the University of Georgia. He attended the University of Georgia for his undergraduate degree in Plant Protection and Pest Management, where he also received an M.S. in Plant Pathology, followed by a Ph.D. in Plant Pathology from Auburn University. He has extensive experience with disease management programs in numerous cropping systems. He serves as the extension fruit pathologist for Georgia – conducting research and technology transfer for multiple fruit commodities. His efforts are directed towards developing IPM practices to solve disease issues and technology transfer of disease-management methods to commercial fruit producers. He also teaches the graduate level Field Pathology Course, the History of Plant Diseases and their Impact on Human Societies Course, team-teaches the IPM Course, coordinates the Viticulture and Enology in the Mediterranean Region Course (Cortona, Italy), and guest lectures in numerous other courses throughout the year.