Experts urge Southern peach growers to remain vigilant, apply copper early
By Vicky Boyd
Blame Mother Nature for providing conditions akin to the “perfect storm” for bacterial spot in Southeastern peaches during 2005.
But whether growers will have to contend with another bacterial spot outbreak as severe this season will depend on weather conditions during leaf and early fruit development, experts say.
“One year does not predict the next,” “It depends on the weather conditions in late winter and early spring.
“If we have favorable weather conditions, then it could be a very severe year again,” says David Ritchie, a North Carolina State University plant pathologist in Raleigh. If we have dry conditions as the trees begin to come out in the spring and the fruit sets, we could expect it to be very light.”
Despite Mother Nature appearing to be in the driver’s seat, Ritchie says growers can take a few steps to avoid being caught off guard again, including using standard pruning practices, following the “2006 Southeast Peach Pest Management Guide” recommendations and watching the weather.
“Be vigilant and watch what is happening next spring as far as moisture, and use copper,” Ritchie says. “If they let the disease get started, there’s nothing that can be done to stop it.”
Phil Brannen, a University of Georgia plant pathologist in Athens, agrees and emphasizes dormant and late-dormant copper sprays before bloom.
“Producers are going to have to use copper applications, even on those varieties that don’t normally get it,” Brannen says. “I think if there would have been any copper out there at all [in 2005], they would have been OK on all but the extremely sensitive varieties, like O’Henry.”
What is bacterial spot?
The organism responsible for bacterial spot, Xanthomonas arboricola pv. pruni, can infect any Prunus species, including peaches, nectarines and plums, and can infect fruit, leaves and current season’s twigs.
Organisms enter leaf scars in the fall and overwinter. When temperature and moisture are conducive in the spring, the bacteria ooze out of natural openings. Leaf and fruit infection typically occurs around petal fall and shuck split.
Bacteria will ooze from stem cankers for about 30 days, but exude from fruit and leaf lesions throughout the season.
Leaf spots are angular and may appear in a shot-hole pattern as the lesion centers drop out. Leaf lesions are more common at the distal end of leaves and around major veins.
Leaves with as few as two or three lesions may turn yellow and drop. During a severe infestation, trees may be defoliated.
Peach fruit infections appear as deep, dark purple to black flecks on the skin, rendering the fruit unmarketable. In smooth-skinned Prunus species, such as nectarines and plums, the lesions have a more water-soaked appearance. Later, the skin is broken, and the flesh below the spot becomes sunken.
Two types of cankers appear on twigs. Spring cankers develop as darkened blisters in early spring near the twig tip of the past year’s growth. The tip injury can be so extensive that the terminal bud fails to open.
Summer cankers appear in mid-season and are irregularly shaped, dark, slightly sunken lesions on the current season’s growth.
Moisture is critical
Wet weather is conducive to bacterial reproduction, and rain and blowing water can help spread the organism. The disease is more of a problem in areas with more than 20 inches of annual rainfall.
Heavy rainfall during prime infection periods coupled with a cool spring that kept fruit susceptible for a prolonged period allowed the bacteria spot organism to explode in 2005. Add to that a sand storm that injured young tissue, providing an infection entryway, and even growers with tolerant varieties had to spray bactericides. Those with susceptible varieties or those caught by surprise experienced crop damage, sometimes severe.
“The thing about this year was the shear number of varieties that were hammered,” Brannen says of 2005. “I talked to producers who have been growing those varieties for 30 to 40 years, and they had never seen it.
“Was it a mutation? A different strain? Did it come in on hurricanes? Is it more resistant to copper this year? I think the answer to all of those questions is ‘no.’ The environmental conditions were just ideal, absolutely perfect.”
Strategies to fight bacterial spot
One strategy to reduce bacterial spot severity is to plant tolerant cultivars. But Ritchie says many growers continue to plant susceptible varieties, such as O’Henry, because of market demands.
Even on tolerant varieties, Extension specialists recommend two to three early-season copper applications as insurance. But growers can typically forego later season treatments.
With susceptible varieties, growers need to be on a strict schedule, using copper initially until the trees leaf out. Then they need to switch to lower rates of copper or oxytetracycline, marketed as Mycoshield and FlameOut, since the plant tissue is sensitive to higher copper rates.
By covering the tree, particularly the woody portions, with copper during the late dormant period, the first thing the bacteria are exposed to as they emerge in spring is the bactericide.
If you can reduce early-season bacteria populations, Ritchie says you may be able to get by with fewer sprays later on.
The challenge with bacterial spot, Ritchie says, is once you see symptoms, it’s too late to treat. Copper or oxytetracycline needs to be applied as a protectant.
One of the questions growers have asked is whether a fall copper application at leaf drop is worthwhile since the bacteria enter through leaf scars. Although the theory sounds good, Brannen says it’s unrealistic because trees drop leaves over several weeks, so growers would have to constantly apply copper to protect the new wounds.