Protection of Apple Fruit from Bitter Rot

Apple bitter rot is a disease that has been an increasing problem in the last 5 years for a lot of growers. This disease is caused by the fungi from Colletotrichum genus and is favored by warm, wet and humid weather conditions prevalent during late spring and the whole summer, but usually occurs when fungicide cover sprays are too far apart while rains are frequent and spray applications do not follow these rain events. The frequent rains we had in the last 4 weeks in northern Virginia were perfect for bitter rot infections. Infections on fruit unprotected by fungicides usually appear in the first week of July, but if conditions in May or July are extremely conducive it is know to appear even earlier (Fig. 1). Occurrence of bitter rot is prevalent in orchard but can spill over to a lesser degree into storage as a postharvest decay. Typical signs of apple bitter rot are very small light brown watery spots under the fruit skin that over time enlarge into 1/4 to an inch size rot lesions that are sunken like saucer in appearance. In other words the fruit loses its round shape, while other fruit rots (e.g. black or white rot) do not cause this flattening of the lesion (Fig. 2). The key telltale sign that a bitter rot is in question is V-shaped rot in the flesh when affected fruit is cut across the lesion while lesions are 1/2 - 1 inch in size (Fig. 3).

Figure 1. Early bitter rot symptoms on ‘Gala’ apple fruit (Photo by Aćimović S. G., 2018)

 

Figure 2. First signs of bitter rot on apple fruit of cultivar ‘Honeycrisp’(Photo by Aćimović S. G. 2019)

 

 

Figure 3. Early bitter rot on ‘Gala’ apple fruit showing V-shaped rot in the flesh (Photo by Aćimović S. G., 2018)

The best ways to reduce inoculum from this disease by cultural practices is removing fruit mummies left over after the winter on the trees following a severe infection last year (this can be done during dormant pruning). Decaying fruit on the orchard floor from last year should also be removed if possible or flail-mowed for faster degradation by soil microorganisms. Bitter rot fungi can also overwinter in fruit buds, but more research is needed to prove that in the growing conditions of Virginia. The most effective option for control of bitter rot are fungicides. Our trials led us to compile a list (see below) of summer cover spray combinations you could rotate, with no more than two consecutive applications of any of these combinations, with focusing in early summer on using weaker fungicides for bitter rot (e.g. Inspire Super; Prophyt) or the ones that leave more visible residue, and going into more effective and less visible residue fungicides:

• Captan 80 WDG 2.5 LB/A + Inspire Super* 12 fl oz/A
• Captan 80 WDG @ 2.5 LB/A + Prophyt** 64 FL/A

• Captan 80 WDG 2.5 LB/A + Aprovia 5.5 fl oz/A to 7 fl oz/A

• Captan 80 WDG @ 2.5 LB/A + Ferbam 76 WDG or Ferbam Granuflo 4.6 lbs/A
• Captan 80 WDG @ 2.5 LB/A + Ziram 6 lbs/A
• Captan 80 WDG @ 3 LB/A + Topsin M** 1 LB/A
• Captan 80 WDG 2.5 LB/A + Flint Extra 2.9 fl oz/A (or Sovran 5 oz/A, or Luna Sensation 5.9 oz/A or Pristine 14.5 oz/A, or Merivon 5 fl oz/A)
• Captan 80 WDG 2.5 LB/A + Omega 13.8 fl oz/A
• Captan 80 WDG @ 3 LB/A

*Under high disease pressure, DMI fungicides and are not effective for bitter rot control.
**Phosphites and thiophanate-methyl are not effective for bitter rot control.

(NOTE: Limit group 11 fungicides to 4 applications per season in total to avoid development of fungicide resistance in Colletotrichum species. This does not mean you can apply 4 applications of each of these materials: Flint Extra, Sovran, Luna Sensation, Pristine or Merivon. It is the opposite, since they all belong to the same group called group 11 fungicides, you can only use 4 applications of any of the fungicides in this group 11, per growing season. You can use either one group 11 active ingredient or alternate different group 11 active ingredients from cover to cover sprays, just as long as their total number of applications is 4 per growing season (whichever is cheaper for your budget). My advice is to save Merivon or Pristine for your last, i.e. fourth spray application before harvest due to their zero days pre-harvest interval of these materials. Do not apply Ziram within 14 days of harvest and Ferbam is not recommended for late-season cover sprays on fresh market fruit due to the potential for dark residual spots of Ferbam at harvest. Do not apply Ferbam within 7 days of harvest).

Captan can be replaced with ziram and/or ferbam when used with Flint Extra, Aprovia, Omega, Inspire Super, or Topsin M. Merivon, Flint, Pristine, Sovran, Luna Sensation, Topsin and Ferbam are effective for control of sooty blotch and flyspeck as well. Your cover spray is worth 14 days or 2 inches of rain, whichever comes first. If you do not get rain for 14 days, you can extend the spray interval to 21 days, under the condition that you do not get rain during the 7 additional days.