By: Kevin Ong and Corinne Rhodes
Peach latent mosaic viroid (PLMVd) was first discovered in peach trees in France in 1976. It is now widespread in Europe, Asia, North America, and South America. Because symptoms are often absent, delayed in developing, or go unrecognized, determining how prevalent it is on a more localized scale is difficult. Regional surveys both internationally and in the United States have found incidences of infection at greater than 50 percent.
The disease is described as latent because infected trees lack early symptoms in the leaves and other visible symptoms are delayed by about 2 years from the time of infection. While a small proportion of plum, apricot, cherry, apple, and pear varieties in parts of Europe, Asia, and Canada are naturally infected, symptoms are not typically seen on these hosts and have never been reported on cherry or apricot. PLMVd is more of a concern for stone fruit growers in areas with more severe strains of the viroid and for those who certify propagation material to be transported internationally. (A viroid is a plant pathogen smaller than a virus and made up of only nucleic acids with a protein coat.)
PLMVd is principally a disease of peaches. Symptoms vary depending on the virus strain and the peach cultivar, but most infections lack obvious symptoms.
The most common symptoms include:
- A delay in blooming (Fig. 1), leaf production, and fruit ripening
- Reduced vegetative vigor
Less common symptoms include:
- Leaf mosaic
- Leaf and fruit blotches
- Flower streaking
- Wood grooving
- Deformed and colorless fruit with cracked sutures (Fig. 2) and enlarged pits
- Bud necrosis
- Accelerated tree aging
- Trees with an open rather than dense growth habit
Symptoms such as flower streaking, wood grooving, and leaf mosaics tend to be sporadic. In parts of Europe, a strain of the viroid causes peach calico, which has more severe symptoms such as fruit blotches and a leaf mosaic that causes white patches over large areas of the fruit and leaves.
While infection by PLMVd alone typically does not cause noticeable or undesirable symptoms, PLMVd-infected trees are more susceptible to both biotic and abiotic (living and nonliving) stresses. The combined effect of a co-occurring infection or stress can increase symptoms and lead to economic losses.
Grafting, pruning, aphids, and pollination can transmit this viroid, but grafting is the most efficient means of transmission. Delayed bloom is a desirable trait in peach-growing regions prone to late spring frosts and grafting with PLMVd-infected source plant materials has been used both intentionally and unintentionally to impart this characteristic to trees. Evidence suggests that there is a low incidence of natural transmission by insect vectors and/or pollination in the field.
Prevention measures are the only management tools currently available for controlling this disease. This includes using only viroid-free material and decontaminating pruning tools with a bleach solution between trees. To help ensure that only viroid-free propagation material is produced and distributed, molecular tools for rapidly detecting the viroid in leaf, bark, and fruit tissues are available.
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