By Kevin Ong and Corinne Rhodes
Bacterial canker of stone fruits is caused by Pseudomonas syringae pv. syringae, a widespread organism that generally causes disease on trees that are stressed. All stone fruit can be affected by this disease but cherries, peaches, and apricots are the most susceptible.
In peach orchards, trees 2- to 8-years-old that are under stress are at the greatest risk. Factors that most commonly predispose trees to infection include:
- Winter or spring freeze damage
- Sunscald damage
- Presence of the ring nematode, Mesocriconema xenoplax
- Light, sandy soils
- Poorly drained soils
- Mechanical injury
- Use of a rootstock not suited to the planting site
- Improper pruning practices (including pruning in fall or winter)
Bacterial canker symptoms vary, but typically occur in the spring when infected buds fail to open and the twigs connected to them die (Fig. 1). Infections on one part of a branch can not only delay bloom and leaf emergence on other, uninfected parts of that branch, but also spread rapidly within it to kill the blooms and leaves once they do emerge. Cankers may occur on larger branches and trunks but will not extend below the soil line. Gummosis (oozing sap) and water-soaked areas on the bark that have a distinct, sour-smelling sap may be associated with cankers. There may also be an increase in the production of water sprouts (shoots that grow from the trunk or branches) (Fig. 2). Bacterial canker can also cause leaf spots similar to those caused by Xanthomonas arboricola pv. pruni, or bacterial spot. These small spots begin as a dark purple color, then become necrotic and develop a shot hole appearance when the necrotic tissue drops out.
In the Southeastern United States, a disease complex known as Peach Tree Short Life (PTSL) causes an overall collapse of trees in the spring. Many buds fail to open and new leaves wilt. PTSL is caused by a combination of winter freeze damage, bacterial canker infection, and the presence of the ring nematode. Nematode infection increases the susceptibility of trees to both cold damage and infection by bacterial canker. This combination significantly disrupts or blocks the vascular system, limiting the delivery of water to affected areas of the tree.
Bacterial canker colonizes the surfaces of healthy plant tissues without causing disease, but can enter into living tree tissues through leaf scars or wounds created by pruning or other means. However, bacterial canker is a weak pathogen capable of spreading and causing disease only in trees that are stressed by some external factor. Healthy, vigorously growing trees are much less likely to be affected.
The rate of the spread of the pathogen within a tree depends on the condition and age of the tree, as well as environmental factors. High rainfall in the fall and winter with temperatures generally remaining above freezing favor the spread of the disease. Depending on the severity of the stress and environmental conditions, the bacterium may be limited to small twigs or, in the most severe cases, may spread rapidly within vascular tissues and lead to the death of the tree within a couple of months.
Applying bactericidal chemicals is generally not recommended or effective for preventing or treating this disease. To reduce the susceptibility of trees to bacterial canker, as well as encourage tolerance to the disease where it is already present:
- Choose a suitable planting site and rootstock. The rootstock can affect susceptibility to the ring nematode, M. xenoplax, associated with PTSL. On sites where this nematode is present,use a resistant rootstock. Guardian has the greatest resistance, followed by Lovell and Viking. Nemaguard rootstock performs poorly in the presence of this nematode.
- Maintain tree health with a targeted fertilization and watering regime.
- Test the soil and leaf tissues annually to determine fertilization needs.
- Remove severely affected trees and prune off dead or dying branches.
- Avoid pruning trees between October and January to help prevent the development of larger cankers in the winter.
- In infected orchards, treat pruning tools with a 10 percent bleach solution after pruning each tree to prevent the spread of the disease.
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