Peach viruses aren’t all equally prevalent across the U.S., with growers in some regions experiencing threats uncommon in other areas. In 1999, Pennsylvania, New York and Michigan growers experienced an outbreak of plum pox virus (PPV), which as of 2019 is considered eradicated from the country. That accomplishment led to a sigh of relief from peach growers, although other peach tree viral diseases remain.
“It’s a big concern,” Elizabeth Cienciewicz, assistant professor of virology, Clemson University, said of PPV during a presentation at the recent Great Lakes Expo. “If it is found, it is extremely devastating. It will kill the tree. There are very harsh eradication procedures that must occur too.”
While PPV is thankfully now not a concern, prune dwarf virus (PDV) is widespread in stone fruits. While not nearly as devastating as PPV, PDV increases a tree’s susceptibility to other pathogens, including preliminary evidence that it can increase susceptibility to bacterial spot.
“The typical symptoms of this really vary, depending on the cultivar,” Cieniewicz said of PDV. The most common, typical symptoms include stunted form, shortened internodes and bark splitting as well as gumming.
The disease is graft-transmissible and can be spread by pollen, making it difficult to keep from spreading from tree to tree or orchard to orchard. It can also be spread via seed, so rootstocks propagated by seed are a potential transmission route. Reduced trunk size, increased watersprouts and fruit yield decreases of up to 40% can happen with PDV infection.
The presence of PDV also increases the likelihood of infection with Prunus necrotic ringspot virus (PNRSV). The initial stage of symptoms – aka “shock” symptoms – is severe. They include severe stunting and shot-holing of leaves. The leaf damage seen with PNRSV is often mistaken for copper damage.
The trees typically recover from these initial symptoms, and the virus can seem to disappear, with trees sometimes making complete recoveries. But the virus remains active in these trees, reproducing and spreading to other trees, at which point symptoms will be visible again.
If PNRSV and PDV are both present in the orchard, a synergistic reaction can occur, leading to a “greater than additive interaction,” Cieniewicz said. “We don’t really know what is going on here at the molecular level.”
The two viruses appear to help each other thrive. Trees infected with both viruses typically have yellowing of leaves and sometimes complete defoliation, are severely stunted, have smaller trunk sizes and drastically reduced yields. When these diseases work in combination the result is known as peach stunt disease (PSD).
“You might not see very noticeable or intense symptoms with either of these viruses in single infection, but there is definitely an additive, synergistic event when you have both viruses together,” she said. “Peach stunt disease is ubiquitous.”
PSD is both graft and pollen transmitted. The results of the interaction of the two viruses differ between cultivars. Spread from tree to tree does occur after several years of being present in the orchard.
Phony peach disease (PPD), caused by a bacterial pathogen and vectored by sharpshooter leafhoppers, has similar symptoms to PSD. It is thought to be graft transmitted in peaches, as it is in other crops. It tends to stay in the roots of peach trees. Decreased yields, trees with darkened foliage, flat tops and with shortened internodes are some typical features of PPD.
Tomato ringspot virus is a nematode-vectored disease that is also persistent in weeds and is widespread. Necrotic areas, thickening of the bark, stem pitting and an overall decline in orchard health occur.
“There is no cure for a virus-infected tree,” Cieniewicz said. “All management with virus diseases boils down to prevention.”
The first step is to plant virus-free trees. The National Clean Plant Network (NCPN) maintains clean fruit tree plant material for cuttings, scions and plantlets, as well as buds, so nurseries and growers can begin with clean planting stock. Using nursery stock from tested sources is vital to keeping orchards virus-free. Crops in the NCPN include fruit trees, citrus, berries, hops, roses, sweet potato and grapes.
The second step is to remove infected trees to prevent secondary spread throughout the orchard or to nearby orchards. Weeds and wild fruit trees can also serve as reservoirs of viruses. Assess the risk from weeds, wild trees and nearby orchards, scout your orchard regularly for signs of viruses and remove diseased trees. Nematodes can also harbor viruses.
Containing virus spread into or throughout the orchard can be difficult. Pollen can readily spread viruses, and pollen itself is readily spread by wind and insects.
“Your vectors are also your pollinators,” Cieniewicz said. “The options for controlling the avenues of spread are pretty limited.”
A grower-led collaboration with Clemson University resulted in the formation of the Southeastern Budwood Program. Researchers test commercial orchards for viruses, taking leaf samples for analysis. Approximately 3,000 trees are tested annually. The rate of infection has been about 5%, primarily with PNRSV.
Tree blocks are mapped out, indicating diseased trees, and shared with nurseries. The nurseries are then able to collect budwood from trees that have tested negative for viruses, and avoid trees with disease as well as those nearby, where the virus may have spread. Growers can eliminate diseased trees from their orchards. Each year, the cultivars tested vary due to industry needs, with the goal of producing clean source material for propagation.
The program began in 2000, and the highest rate of infection occurred in 2001, with about 13% of the tested trees having viral infections. The rate dropped steadily through 2014, when less than 2% of the trees tested were virus-positive.
Despite this success, there has been a recent increase in infected trees, and PDV was detected in an entire block of tested trees. It is believed that untested trees in close proximity to known infected ones were utilized for propagation purposes at some point, and must have been infected as well, ultimately spreading the virus.
“It definitely has overall reduced the virus pressure in this area,” Cieniewicz said of the program. “With the virus-infected trees, [growers] very often see higher susceptibility to some other pathogens.”
Virus pathogens of stone fruit can cause devastating symptoms, or can be less destructive. But synergies exist, and one pathogen often leads to further issues. Nipping primary pathogens in the bud can prevent much bigger future concerns.