Except for the fruit ripening stage, the petal fall (PF) through second cover (2C) stages on peach and nectarine constitute perhaps one of the most important periods for disease control. In addition to continued control of blossom blight at petal fall, three other diseases must be considered as well: rusty spot, scab, and bacterial spot. Scab and bacterial spot alone can cause considerable crop loss under favorable conditions. Furthermore, new data (2017) are indicating that latent brown rot infection of green fruit prior to pit-hardening may be more significant than previously thought; more on this topic as the research continues.
Of course, all of these diseases do not need to be controlled in every peach block. Orchards that have not had occurrence of rusty spot in past years, either because of resistance or lack of local inoculum, obviously do not need to be treated. Similarly, only those cultivars moderate and highly susceptible to bacterial spot should be sprayed for this disease (see NJ Commercial Tree Fruit Production Guide, CTFPG, for cultivar susceptibility table). However, in general, all blocks do need to receive fungicide for scab control.
Scab. Proper timing for scab fungicide applications consists of an early shuck-split spray (<5% shucks split) followed by subsequent cover sprays at 7-14 day intervals. Use shorter 7-10 day application intervals if rains are frequent, longer in drier weather. Since scab inoculum production on twig lesions ends during early-to mid-July in New Jersey (approximately 100 days after 1st bloom), regular cover sprays should be applied until this time. Given that fruit infections have a minimum 40-day incubation period, one might be tempted to halt sprays even sooner on early maturing cultivars. However, this practice should be avoided since infection of new shoots will still occur as long as inoculum is available.
If scab has not been a problem in a block, a good standard maintenance program consists of Bravo at shuck-split followed by either Captan or Sulfur cover sprays. However, if much fruit scab occurred last year in the block, then it is likely that many more twig lesions are present to produce inoculum for this year’s epidemic. In this case, an application of Gem at maximum rate (3.8 fl oz/A) at petal fall is highly recommended. This one application will control the “tail end” of blossom blight, provide good initial rusty spot control, and act as an anti-sporulant to inhibit scab inoculum production on twig lesions.
If a Gem application is applied at petal fall, then it should be followed by Bravo at shuck-split and, preferably, Captan cover sprays. A follow-up Gem or Quadris Top application at first or second cover (in place of the Captan) should be considered if scab pressure is extremely high and/or frequent rains are occurring. The remaining cover sprays should be Captan. Quadris Top is currently the most effective scab protectant available; however, label warnings must be followed carefully since it contains azoxystrobin, a fungicide toxic to 40% of apple cultivars.
It should be noted, for resistance management reasons, that the enhanced scab program need not be and should not be used every year. Once the scab twig lesion levels have been reduced, the standard Bravo / Captan or Sulfur program should be sufficient for scab control. The QoIs in Gem and Quadris Top are rated as having a high risk for resistance development, so overuse needs to be avoided.
Rusty spot. Management of rusty spot on moderate to highly susceptible cultivars (see cultivar susceptibility table in CTFPG) requires fungicide applications at PF, SS, 1C, and 2C. The most effective fungicides are the DMIs Rally 40WSP and Rhyme 2.08SC. Rally at 3 to 4 oz/A is recommended for moderately susceptible cultivars, while 5 to 6 oz is best for highly susceptible cultivars. In recent field studies, Rhyme at 6.5 fl oz (labeled rate is 7 fl oz/A) has provided excellent control, equivalent to Rally at 5 oz/A.
In a standard scab program, either Rally or Rhyme should be added to the Bravo and Captan sprays, which lack rusty spot efficacy. If Gem or Quadris Top is applied for scab, then these materials will also provide “good” control of rusty spot, so the Rally or Rhyme will not be needed at these timings.
Bacterial spot. The three to four week period following early shuck split is a critical time for primary infections of peach or nectarine fruit. Fruit lesions initiated during this period tend to be large and deep. However, fruit of moderate to highly susceptible cultivars continue to be susceptible well into the summer. Lesions occurring during this time are smaller, more shallow, but often quite numerous. Either way, much fruit can quickly become unmarketable if control measures are not employed.
Over the years, many different experimental bactericide treatments have been examined for control of bacterial spot. In each of these studies, an oxytetracycline (OTC) antibiotic standard (FireLine or Mycoshield) and copper standard (e.g., Kocide 3000, Badge X2, etc…) have been employed. Unfortunately, few new highly effective materials have been discovered. However, the most consistent control occurred when either the OTC or copper standards were applied on a weekly basis, beginning either late petal fall (100%) or early shuck split (<5%). For example, in 2016 Kocide 3000 30DF at 1.7 oz/A and FireLine 17WP at 1.5 lb/A yielded 85% and 88% marketable fruit, respectively. Sprays were applied via airblast at 100 gal/A, or approximately 50% full dilute. Lower spray volumes, while acceptable for some fungicide sprays, are not recommended for bacterial spot. Also, spray timings within each week were adjusted so that applications were made immediately before any given rain event.
Given the equal efficacy of OTC and copper bactericides, the most logical seasonal program would be to alternate the two materials, beginning with the copper at late petal fall or very early shuck split. This alternation program has several advantages over a solid OTC program, solid copper program, or OTC + copper mixture program. First, given the lower cost of the copper, the alternation program will be less expensive than the solid OTC program or OTC + copper mixture program. Second, alternation is an accepted strategy for resistance management; tetracycline antibiotics are rated as having a high risk for resistance development. Finally, the alternation program will sustain less foliar phytotoxicity and defoliation than the solid copper program or copper + OTC mixture program. For example, in the 2016 study, the Kocide only treatment sustained 31% defoliation, while the FireLine treatment had only 12% defoliation.
Rates of application for materials discussed above are provided in the New Jersey CTFPG. In general, higher rates are needed when inoculum and/or environmental conditions favor higher disease pressure. And, as always, followed product labels.
