Peach and nectarine orchards in New Jersey continue to sustain considerable shoot death and fruit loss from constriction canker, caused by the fungal plant pathogen Phomospis amygdali. Many of our research orchards at RAREC this season had an above average number of cankers. The majority of infections that took place last fall and this past spring have girdled the shoots, resulting in dead or blighted branches. Any fruit distal to the cankers on these shoots have already fallen off or remain attached, slowly drying up and shriveling. This yield loss directly impacts your bottom line.
Breaking the Disease Cycle
Cankers on infected shoots are the source of inoculum. Spores from these cankers will initiate infections on this year’s new growth during the post-harvest leaf drop period and spring bud break. These newly infected shoots, which are to bear fruit next season, will then die next summer, producing yet another “crop” of cankers to continue the disease cycle.
Two control methods should be combined to halt the disease cycle. The first method is to remove as much inoculum as possible, thereby reducing the likelihood and amount of infection that does take place. The second method is to protect the shoots by applying fungicides during any potential infection periods during the fall and spring. Given that neither of these approaches are highly effective (>90% control), both methods should be employed to obtain maximum control. Both steps are critical if one wishes to rehabilitate moderate to heavily infected orchards back to good health.
Cultural and Labor Management
The obvious way to reduce inoculum is to prune out any cankered shoots. The best time to perform this function is late summer. By this this time, the majority of cankers have killed the shoots so locating the cankers is much easier
Pruning out diseased shoots takes a lot of man-hours and some training. Since most labor at this time is geared towards harvest, focus your control efforts on the most valuable and/or heavily infected blocks. Make sure workers cut 2-3 inches below the canker. I have seen cuts above the canker! The dead shoot was gone, but the sporulating canker remained! And, finally, prune during dry weather so that the cuts have time to heal before other pathogens can enter the tissue.
Fall Infection Process
During fall leaf drop, which occurs from September through late November in New Jersey, the normal abscission of leaves exposes leaf scars to the environment. Since temperatures are becoming cooler during this period, plant growth is slowing down, and more time is required for leaf scar suberization and periderm formation. Consequently, the scars remain susceptible to invasion by pathogens for a longer period of time than under warmer conditions (e.g., summer).
During rainy periods the pathogen produces spores in fruiting bodies embedded in the cankers. These spores are disseminated by rain-splash and wind-blown rain to fresh leaf scars. After entering the twig tissue through the leaf scar, the pathogen begins to colonize the surrounding tissue. Sometimes, by late fall or early winter, very small slightly sunken reddish-brown cankers can be seen surrounding infected leaf scars. These cankers continue to develop and enlarge during any warm periods in the winter and into the next spring and summer. Eventually, the canker girdles the twig, causing the classic shoot blight symptom and direct fruit loss.
A variety of different fungicide chemistries have been examined for management of constriction canker. The most effective fungicides were the two protectants chlorothalonil (Bravo) and captan. Even so, they yielded only 46-71% control, much lower than the >90% control we have come to expect from fungicides. This outcome explains why it is important to also perform canker removal, which alone reduced canker incidence as high as 42%.
Fungicide applications for constriction canker control should begin in the first or second week of September and continue at 10-14 day intervals until all leaves have dropped, generally by the third or fourth week in November. At 14-day intervals, a total of six to seven fungicide sprays will be applied. This program is recommended only for moderate to severely infected orchards, not as a general purpose spray program for all orchards. The idea is to maintain fungicide protection on susceptible leaf scars throughout the leaf drop period.
Most fungicides, including captan, are not labeled for multiple applications during the fall leaf drop period. However, a FIFRA Section 24(c) special local need label was obtained for application of Bravo WeatherStik on peach in New Jersey (no other state has this label). Applications are recommended at 3-4 pints/acre, preferably at high enough volume to get good coverage of leaf scars (100 gpa best). The only restriction is that no more than 20.5 pints can be applied per acre per year. Thus, if you applied 3 pt/A this year at shuck-split, then you have 17.5 pts available for fall sprays, which translates into six sprays at almost 3 pt/A each. If you need an extra application, you can include a single spray of Captan 80WDG at 3.5-5 lb/A. Of course, when implementing this program, a separate leaf curl spray will not be needed.
The above program is designed for rehabilitation of moderate to severely infected orchards. Several years of the program may be necessary to bring an orchard back to a healthy state. Once that state is achieved, then constriction canker control tactics can probably be reduced to a “maintenance program”, which could also be useful for keeping all orchards of susceptible varieties canker-free. Unfortunately, however, no research has been conducted to determine the minimum control needed for such a program. Nevertheless, one possible recommendation would be to continue with canker removal, but reduce Bravo applications to once per month (Sep, Oct, & Nov) at the higher 4 pt/acre rate.