Franz Niederholzer, UCCE Farm Advisor in Colusa and Sutter/Yuba Counties & Themis Michailides, UC Davis Plant Pathologist

Cytospora, bacterial canker or Botryosphaeria canker infections limit production and shorten the life of a prune orchard. Cytospora and Botryosphaeria cankers are caused by fungi, such as Cytospora and Botryosphaeria species, while bacterial canker is, of course, a bacterial disease. Recent research in Dr. Michailides’ lab provides information growers can use to help manage Cytospora and Botryosphaeria. Rootstock trials in Yuba and Butte Counties may provide information on selections that limit future bacterial canker infection. This article briefly reviews this information and suggests practices to limit infection from these damaging diseases. While the times of greatest infection risk are late and early in the year, when pruning wounds are exposed to rain, now is a good time to cut out infected wood and plan to limit future infection risks before the harvest season arrives.

Disease infection requires the presence of a pathogen, a vulnerable host and certain environmental conditions, including an extended period of moisture on the host.

The pathogen, especially Cytospora, is present all the time. Molecular approaches detected the DNA of both Cytospora and Botryosphaeria species in young trees, however, more research is needed to determine whether the presence of DNA of these pathogens in tissues of prunes with no symptoms (“latent infection”) can lead to symptomatic Cytospora and/or Botryosphaeria canker diseases.  Cytospora infections can be found in virtually all mature prune orchards in California. Spores of these fungi, as well as other pathogen species, were found in rainwater collected within the canopies of prune orchards in Sutter and Yuba Counties in the past two winters.

Cytospora and Botryosphaeria cankers infect prune trees through breaks in the protective bark caused by sunburn, bacterial canker, and/or pruning wounds. Pruning produces multiple disease entry points, while moisture (rain) provides the environmental conditions for infection. Severity of canker development in pruning wounds — made the first week of March — is highest in spring (March) and generally decreases through the summer. Trials in Yuba County over a two year period show that Cytospora canker infections of pruning wounds can be significantly reduced when Topsin^®-M fungicide is applied after pruning. Topsin^®-M and Rally^® WP now have 2EE labels for treating pruning wounds in stone fruit and almond.

Fungicides are generally considered effective for about 2 weeks after application. Pruning just ahead of rain on young trees without protecting the pruning wounds with an effective fungicide is especially risky. Pruning followed by an extended dry period should provide less risk for infection. Growers with young trees interplanted in mature blocks should be especially careful to schedule pruning of interplants when no rain is in the forecast and treat with Topsin^®-M or Topsin^®-M and Rally^® WP after pruning and before rain. (Rally^® WP, alone, has not best tested for pruning wound canker control in UC trials on prune trees.) Check with your PCA regarding rates, timing, and application methods. Read the label, carefully, before treating. Research on managing fungal cankers in prunes continues in Dr. Michailides’ lab with funding from the California Dried Plum Board.

The bacterium causing bacterial canker infects prune trees weakened by root zone stressors including ring nematodes (the biggest factor) as well as low nitrogen status. Commonly, Cytospora infections can follow bacterial canker infections, providing a lethal “tag team” effect on orchard health. There are no known “silver bullets” to control or protect stone fruit, including prunes, from bacterial canker. Soil fumigation, if ring nematodes are present, is highly recommended. Rootstock selection is an important piece of bacterial canker management, with Lovell, Viking, Atlas and Krymsk 86 showing very good survival in the current Yuba County UC prune rootstock trial, while plum roots (Myro29C, M2624 and Myro seedlings) show significant losses of up to 50% in some reps. M40 has shown better survivability than traditional plum roots, but some losses have occurred.

One practice to limit, but not eliminate, bacterial canker in stone fruit is the use of fall urea sprays. Research by Roger Duncan, UC Farm Advisor in Stanislaus County, has shown a late October application of a HIGH rate of low biuret urea (100 lbs of urea in 100 gallons of water) significantly reduced the spread of bacterial canker in trees of a 2^nd leaf peach orchard on a very sandy soil. This research has not been repeated in prunes. Whether this treatment might be effective in prunes, as well as whether standard urea (with a low biuret level) could be used safely instead of low biuret urea (and at what rate) is unknown.

Progress is being made in understanding practices to limit damage from canker diseases in prune orchards. While the times of greatest infection risk are after harvest (fall through spring), we suggest that growers and PCAs begin planning now to manage canker diseases postharvest.