Effective management of bacterial spot on peach and nectarine necessitates application of bactericides during the post-bloom cover spray period. Early fruit infections during the first two to three weeks of this period generally result in large, deeply pitted blackened lesions that often ooze with sap. Later season fruit lesions are less pitted and much more shallow, but can be numerous, particularly on highly susceptible cultivars. Either way, considerable yield loss can occur on susceptible cultivars when the environment is favorable and/or inoculum levels are high.
Bactericide applications typically begin at late petal fall or early shuck-split stage and continue on a 7-to-14-day interval throughout the summer. Shorter application intervals should be used when rainy periods are frequent and temperatures range from 75F to 85F. A longer 14-day interval is acceptable during extended periods of dry weather.
All highly susceptible cultivars and most moderately susceptible cultivars need to be treated in most years. Although no cultivars are considered immune to bacterial spot, those listed as having low susceptibility often do not need bactericide sprays. Table 11.1 on page 194 in the 2014 NJ Commercial Tree Fruit Production Guide lists the susceptibility of 160 peach and nectarine cultivars. These ratings were based on data mostly from southern New Jersey.
The organometallic bactericide Tenn-Cop 5E, a mixture of three copper fatty acid salts, was the copper standard for cover spray applications for many years in New Jersey. This product was typically applied at 4.0 to 8.0 fl oz/A, which provided 0.25 to 0.5 oz of copper per acre (referred to as “metallic copper”). Today, in the absence of Tenn-Cop, a variety of inorganic and organometallic copper materials can be applied to peach and nectarine based on their labeled preharvest interval (PHI) of either 0 days, 21 days, or second cover.
The table below lists these products along with the recommended rate/A for cover sprays which provides 0.5 oz copper/A (considered to be 1X rate). Note that most of the product labels have much higher rates listed for either dormant application or for just two applications at first and second cover. These rates are too high for a multiple cover spray program and will result in excessive leaf injury and defoliation. Note Cuprofix Ultra and C-O-C-S are not included in the table because they are labeled for use only through shuck split and petal fall, respectively.
|Copper Bactericides for Peach Bacterial Spot Control|
|REI||PHI||Post-bloom Application Rate/A|
|0 Day PHI|
|Kocide 3000||30DF||Copper hydroxide||48 hr||0 days||4.0 – 8.0 oz||1.7 oz|
|Cueva||0.16F||Copper octanoate||4 hr||0 days||0.5 – 2.0 gal||25 fl oz|
|21 Day PHI|
|Nordox||75WG||Cuprous oxide||12 hr||2nd cover||10.7 oz||0.7 oz|
|Nu-Cop||50DF||Copper hydroxide||48 hr||21 days||1.0 – 3.0 lb||1.0 oz|
|Mastercop||0.54SC||Copper sulfate pentahydrate||48 hr||21 days||4.0 – 8.0 fl oz||7.4 fl oz|
|Champ Formula 2 Flowable||2.93F||Copper hydroxide||48 hr||21 days||none provided||1.4 fl oz|
|COC DF||50DF||Copper oxychloride||24 hr||21 days||1.0 lb||1.0 oz|
|Copper-Count-N||0.77F||Copper diammonia diacetate complex||48 hr||21 days||1.0 qt||5.3 fl oz|
|Badge X2||28DF||Copper oxychloride + copper hydroxide||48 hr||21 days||8.0 oz||1.8 oz|
After applying two-to-three applications of one of these copper products, circular reddish-brown spots should be visible on the oldest leaves. This injury is normal, to be expected, and indicates that the proper amount of copper is present to provide control of the bacterial spot pathogen. Eventually, the leaves will drop, most likely from a combination of bacterial spot lesions and copper injury.
Defoliation will occur from the base of the shoot outward toward the tip. As long as the shoot continues to grow and produce new leaves, an adequate amount of photosynthetically active tissue should be present to produce a crop of fruit. The goal is to apply enough copper to protect the fruit, but not so much that leaf drop exceeds leaf production. For this reason, copper bactericides should only be applied to trees with moderate to vigorous growth; older and/or slower growing trees may not produce new leaves at a high enough rate to compensate for leaf loss.
Coverage is critical for bacterial disease control. The recommended minimum volume for all of the listed copper products is 100 gallons per acre, preferably as full sprays. Alternate row middle (ARM) sprays are acceptable as long as they are applied at twice the frequency as full sprays (e.g. a 10-day full spray interval is equivalent to two ARM sprays at 5-day intervals). Note that if an infection period is approaching, both half sprays should be applied prior to the event.
Product Choice and Rates
A two-year field study examining four copper bactericides having different active ingredients (Kocide 3000, Cueva, Badge X2, and Nordox) was recently completed in a highly susceptible O’Henry peach orchard. In this study, sprays were applied every 6 to 11 days starting at shuck split at both 2X and 4X rates.
When applied at the same rates of actual (metallic) copper per acre, few significant differences in efficacy were observed among the four copper bactericides. That is, all four products performed equally well in controlling fruit infection; 74 to 80% of fruit were marketable (fruit grades 1 and 2). Thus, it does not appear that choice of copper bactericide is important for control. Other materials have been examined in the past, but so far no one product appears “best” in terms of efficacy.
Although differences were not detected among the four copper bactericides, a significant rate effect was observed in both years of the study. Application of the higher 4X rate significantly increased the total amount of marketable fruit or fruit in grade 1 relative to the lower 2X rate. Interestingly, the level of defoliation was not significantly different between the 2X and 4X rates. In general, low rate treatments allowed more leaf loss from bacterial spot, while high rate treatments better controlled bacterial spot but simultaneously caused more leaf loss from phytotoxicity.
The take home message from the above study is that higher rates decrease the amount of fruit infection resulting in an increase the amount of marketable fruit.
While a 4X rate is not currently recommended, some applications at the 2X level may be worth trying, particularly before major infection events. To determine the 2X rate, simply double the recommended 1X rates listed in the table above. For example, the 1X rate for Cueva is 25 fl oz/A, so the 2X rate would be 50 fl oz/A, which provides 1 oz of copper per acre. When applying at this higher rate, the shoots should be closely monitored for any increase in defoliation.
The focus of this article was on copper bactericides. However, the application of the antibiotic oxytetracycline is also recommended for bacterial spot control of peach and nectarine. FireLine and Mycoshield are the two oxytetracycline products available for stone fruit.
The recommended rate for both products is 1.5 lb/A at 100 gallons/A volume. As with copper bactericides, higher volumes are critical for good coverage. Given the short residual activity, ideal application timing is within 24 hours prior to an infection event. The incorporation of oxytetracycline into a copper program helps reduce defoliation since the antibiotic is not phytotoxic to foliage.