Over the years the American peach and plum industries have had serious problems with nasty plant pathogens. The really bad diseases are caused by systemic viruses, and more exotic pathogens such as phytoplasma and viroids — very tiny micro-organisms that cannot be eradicated from a tree in the field and get worse each year. The only cure is to destroy the tree. The strategy is to keep the tree from being infected in the first place. Turns out the strategy differs with the type of disease, and depends on cooperation to get it done.
Kill The Patient
The peach yellows epidemic of the late 1800s was the first bad experience for early fruit growers with these exotic systemic diseases in the U.S. Growers began noticing yellowing dwarfed leaves, weak trees, bitter fruit, and eventual tree death. Not understanding the situation, growers attempted in vain to control the problem with infusions of various substances such as wood ashes, salt, potash, and lye.
The only approach that worked was to simply destroy trees displaying the symptoms. It was eventually discovered that heat treatment was effective for eradicating the pathogen from nursery stock. With clean nursery stock growers could start with disease-free trees.
Lurking In The Woods
A closely related problem is X-disease of peach and cherries. Like peach yellows, the X-disease pathogen can spread from tree to tree by specific types of leafhoppers. What makes X-disease different from peach yellows is that X-disease has become established in many parts of the U.S. in wild hosts such as chokecherry, wild cherries in surrounding hedgerows, and woods.
In this case, management requires clean trees from nurseries, eradication of infected fruit trees but also elimination of potential wild hosts within at least 500 feet of the orchard. Here, collaboration among growers to find and destroy wild hosts in the nearby forests and hedgerows is critical to managing the disease. One of the fortunate aspects of the disease is that infected peaches and most sweet cherries look sick within a few years of infection and can be found and destroyed.
Dealing With Hidden Pathogens
A more troublesome disease of recent years is plum pox virus (PPV) disease of peach, nectarine, apricot, and plum. The “D” strain of this disease was detected for the first time in the U.S. in 1999, and has appeared in several states including Pennsylvania, Michigan, and New York, as well as becoming a major problem in Ontario, Canada.
With PPV, trees can become contagious shortly after infection, but take up to three years to show symptoms, with some varieties remaining symptomless carriers. What makes PPV a particular problem is that it is transmitted by several species of aphids that can build to relatively high numbers in orchards and fly to new hosts. In addition, hosts such as wild plum can serve as reservoirs for the pathogen.
Like X-disease, once PPV gets established in wild hosts, eradication from an area becomes very difficult. Since PPV-infected trees can remain symptomless for years, nurseries had to quickly change how they obtained their budwood for tree propagation. For example, Adams County Nursery in Aspers, PA, now requires that all budwood for stone fruit propagation must come from trees propagated from budwood thoroughly tested by the National Clean Plant Network facility in Prosser, WA.
Surveys are important for finding hidden pathogens. A key partner for keeping exotic pathogens under control in the U.S. are the various state departments of agriculture. For example, repeated annual surveys for PPV by the Michigan Department of Agriculture and Rural Development was credited for finding a single tree in 2006 infected by PPV. The state departments of agriculture in Pennsylvania and New York have played similar important roles in dealing with PPV in those states. A good nursery disease testing and certification program goes a long way to managing these cryptic diseases.
Do You Feel Lucky?
As Clint Eastwood says in “Dirty Harry,”: “You’ve just got to ask yourself one question: ‘Do I feel lucky?” Fueling the problem of contaminated budwood is the scarcity of tested budwood and strong demand by growers for new trees. This increases the temptation for nurseries, growers, and other propagators to take shortcuts and make new trees using budwood from commercial orchards or other less protected sources. The danger lies in propagating from poorly tested, non-isolated budwood sources such as backyard trees or commercial orchards, or worse yet, from sources out of the region or country. The risk may be low, but the cost of contaminated trees can be huge.
An excellent approach to handling the problem of dirty budwood is the Southeastern Budwood Program, under direction of Dr. Simon Scott at Clemson University. This program, part of the National Clean Plant Network, is funded by USDA, growers, and nurseries to use a rigorous system to test potential budwood orchard blocks for target pathogens. Budwood from the tested blocks are used by three Tennessee nurseries and by cooperating growers to propagate trees.
Another source for tested budwood of stone fruit and almond is Foundation Plant Services (FTS) at the University of California, Davis. Also part of the National Clean Plant Network, regular testing and visual inspections at FPS help retain the certification of the trees and rootstock seed.
A sufficient source of “clean” budwood requires advanced planning and a support system for getting it done. Being prepared is better than relying on luck.