Over the past few weeks, there have been a high frequency of problems reported with bacterial canker and blast. Bacterial blast results in blighted blossoms and causes crop loss. Interestingly, when there is a high frequency of blast, the very similar disease of bacterial canker appears a few weeks later in young trees(we are starting to get reports of canker now). Both of these problems are complicated. They involve the bacterium Pseudomonas syringae, plant stress which is often mediated by nematodes, and the weather.
P. syringae is an interesting bacteria. It lives naturally on plant surfaces as an epiphyte – meaning that it generally does not cause problems. During wet winters which create a favorable environment, bacterial populations increase and spread through the tree and orchard. At some point the population becomes large enough that it begins to invade plant tissues – starting with buds (bud drop), then flowers (blast), and finally bud scars and other openings which leads to bacterial canker.
Tree health has a lot to do with the susceptibility to P. syringae infection. Generally, if the tree is healthy, infections rarely occur. Weak, young trees- usually three years or less- are susceptible to bacterial canker which can cause the loss of tree scaffolds or death. Mature orchards usually don’t exhibit bacterial canker symptoms, but rather have blast and bud drop problems. Orchards with severe problems are typically planted in sandy soils, which often tend to be acidic, hold less water and nutrients as well as being more conducive to plant parasitic nematodes. All of these factors, if managed improperly, can increase tree stress, increasing susceptibility. Some micronutrient deficiencies, notably iron, have been found to increase susceptibility, while balanced, proper major nutrient fertilization has been shown to reduce occurrence. Severe water stress can also increase occurrence, and is thought to be due to the reduced carbohydrate reserves within the tree.
Nematodes (especially Ring nematode) and other replant problems have been linked to an increased occurrence of bacterial canker, blast, and bud drop. Susceptible rootstocks (e.g. peach-almond hybrids) should not be planted within soils in which ring nematode is present or expected. Resistant or tolerant rootstocks should be considered within these soils. Localized test plots have found that Viking rootstock generally has reduced bacterial canker occurrence in comparison to Nemaguard or Hansen 536. The data on other peach-almond hybrids is unclear.
Managing bacterial bud drop, blast, and canker is difficult and there are no “silver bullets.” Pre-plant soil fumigation is recommended to assist in nematode suppression and to help increase young tree vigor. The use of Viking rootstock should also be considered. Proper irrigation and fertilization practices should be implemented to increase growth and vigor. Lo biuret urea sprays made in the fall help reduce canker size. Copper containing sprays could also be made, but the reviewed data is a bit unclear on the consistency of copper efficacy. Generally, a late fall and a mid-dormant spray seem to reduce canker and blast occurrence. Post-plant nematicides have helped in reducing bud drop in some locations, but the efficacy of many post-plant nematicides varies orchard-to-orchard. Later pruning also seems to reduce canker occurrence. Specifically for blast, the use of effective frost control practices can reduce the severity of blast and should be implemented. Generally, as many practices as possible to improve tree vigor should be implemented.
More information can be found at the UC IPM website entry on bacterial canker.