Bitter Rot

In 1907, Thomas Burrill wrote that regarding bitter rot, “There was nothing to do but to helplessly submit to the inevitable.” However, he continued and stated “that what formerly seemed incomprehensible is easily understandable.” Some key points he touches on regarding the management of bitter rot are weather conditions and sanitation. Obviously, there is little that can be done about the weather. The bitter rot pathogen complex grows well in hot, wet weather.  I say complex because it turns out there are several different species of Colletotrichum causing this disease (or causing Glomerella leaf spot). In fact, even the complex is complex, with the previous delimitation of Colletotrichum gleosporioides and C. acutatum both now described as complexes consisting of several different species each!

What is interesting, 110 years later is that little has changed by way of management options: “(1) cutting of and handpicking the cankers and old mummies; and (2) spraying with Bordeaux mixture”. We have more options than Bordeaux, but first, let’s focus on sanitation. Burrill points out that if you could remove every limb and twig infection (which may not be possible) and then take off all old infected fruits (which is possible) the disease would be controlled “in ordinary cases”.

Since removing every limb and twig infection is not possible, fungicides must be used “so as to prevent the penetration of the germ-tubes of the spores. This means actual coating of the apples with the germicidal substance…” Although there have been scattered reports of Glomerella leaf spot throughout the state (Fig. 1), our main problem is the fruit rot (Fig. 2). Our next problem is the disconnect between infection and symptom development: Fungicides to protect against bitter rot (and probably the other summer rots, as well) need to be applied beginning at bloom—at the latest. Recent work by Villani and Nance (2017) found at ~30% incident of bitter rot when applications began at petal fall. Early studies pointed to petal fall as the time to begin spraying, but the data by Villani and Nance clearly show that you need to spray early and often to protect against bitter rot.

But what to spray? Work by Brannen et al. (2017) found that early applications of mancozeb for as long as possible (within the 77-day PHI), followed by a rotation of Captan 80W(5 lb) and Inspire Super or Captan 80W(5 lb) and Merivon from petal fall to 6th cover were found to provide the best levels of bitter rot control (Aprovia + Captan was also found to be effective in this study, but has a 30-day PHI). It is important to note that Prophyt was used in this tank mix, too. It would be interesting to determine if Prophyt in particular is necessary for this result, or if any acidifier would be sufficient.  We are collecting data at Meigs this year looking at Rally and Pristine included in cover sprays to see if less costly options are equally effective. We hope to have that data available to you by Hort Congress. So far, our bitter rot and white rot infection pressure seems to be quite high; it will be interesting to see if there is a difference between treatments.

Finally, way back in 1907, Burrill concluded that it is still best to choose varieties not so susceptible to the disease. Susceptible varieties include perennial favorites like Cameo, Cripps Pink/Pink Lady, Fuji, Gala, Ginger Gold, Granny Smith, Grimes Golden, Honeycrisp, Ida Red and Nittany. High value cultivars, like Moneycrisp, may warrant the costlier fungicide applications.

Literature Cited
Brannen, P, and R. Covington, D. Rogers. 2017.Evaluation of fungicide programs for bitter rot of apples in Georgia, 2016. PDMR 11:PF022.
Villani, S.M., and D.A. Nance. 2017.Evaluation of Merivon timings for the management of Glomerella leaf spot and fruit rot on ‘Gala’ apple in NC, 2016. PDMR 11:PF011.
Burrill, T.J. 1907. Bitter rot of apples. Available on line at: https://www.ideals.illinois.edu/bitstream/handle/2142/3142/bitterrotofapple07burr.pdf?sequence=1

 

 

 

 

 

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