Orchard Considerations August 2019

In prune production, a new mantra has become “some years are bacterial canker years, while every year is a Cytospora year”. Bacterial canker infections are caused by Psyudomonas syringae (same bacterium that causes bacterial blast in almond). While P. syringae is ubiquitous across surfaces in the orchard, it only causes infections and damage in certain years, under the right environmental conditions. Wet and cold springs are conducive to bacterial canker infections, which although severe and often lethal, die out as the weather warms and do not continue to spread the following year (i.e. annual disease, not perennial). Going into this spring, we knew the wet and cold conditions in late winter could mean that we were likely headed into a bacterial canker year. However, diagnosing the dieback (cankers) we observed this spring brought some surprises. One surprise was observing dieback at the tops of trees associated with horticultural oil use during dormancy.

For more information and photos of canker observations in 2019, see: sacvalleyorchards.com/photos-from-the-field/spring-cankers-in-prune/

Oil Damage. Some tip dieback at the tops of trees was observed in many orchards this spring. This discrete dieback of upright branch tips occurred in orchards that received a dormant horticultural oil application and is consistent with how oil burn presents (photo 1). Oil was applied to this block during dormancy (2-3 gallons December/January). Although oil can help provide effective control of scale insects, drying weather before application (e.g. a drying north wind) can lead to phytotoxicity “burn” and dieback.

Bacterial Canker. Although not every year is a bacterial canker year, we found on our spring farm calls, that 2019 was indeed one of those years. As we expected, the wet and cold conditions in late winter appeared to be conducive to infections. For example, in one orchard select patches of trees showed the extensive dieback consistent with bacterial canker (photo 2). Bacterial canker was very likely the cause of this dieback due to the tell-tale signs of flecks (photo 3), as well as the fermented/sour smell associated with the sour sap phase of bacterial canker decline. Other stressors in combination with the wet/cold late winter, predispose trees to bacterial canker. Typical predisposing factors include ring nematode, sandy or low pH soils, clay/shallow hard pans, and low nitrogen. Therefore, alleviating stressors where possible by keeping trees healthy and vigorous is key. Other possible management strategies include spot fumigation for ring nematode and rootstock selection. A late October application of a HIGH rate of low biuret urea reduced the spread of bacterial canker in young peaches but is untested in prunes. You can read more about managing cankers at: sacvalleyorchards.com/prunes/diseases-prunes/managing-canker-diseases-in-prunes/

Cytospora. Cytospora infections can be found in virtually all mature California prune orchards. Predisposing damage that allows for Cytospora, as well as Botryosphaeria canker infection, include breaking of the bark from sunburn, potassium dieback, bacterial canker, ring nematode, and pruning wounds. Canker infection through pruning wounds is an especially great concern when there is mechanical hedging or topping that make thousands of indiscriminate pruning wounds that are potential entry points for rain-splashed fungal spores. For example, in one orchard that was mechanically boxed in the fall of 2017, Cytospora was subsequently diagnosed during the following 2018 bloom when some trees were not flowering out, except low in the canopy (photo 4). Because Cytospora cankers are perennial and will continue to grow and spread unless they are cut out, one year later during the 2019 bloom, the canker on one tree had spread all the way down to a primary scaffold (photo 5).

In this particular orchard the hedging cuts in the fall of 2017 are presumed to be the entry point of Cytospora infections during subsequent rainfall events. The severely affected trees in this orchard were on the most vigorous rootstocks (e.g. Atlas) which would have had the largest cuts made to them when mechanical (boxed) hedging cuts were made. Approximately 30 days of dry conditions are required after a pruning cut is made for the surface to callus over and not be susceptible to infection, and during this window larger cuts remain susceptible to infection longer than small cuts. Cutting out cankered wood during the dry period late in the growing season and early in the post-harvest period, is the only method of stopping the spread of damage in the tree. Cutting out infected wood is also critical for reducing the production of inoculum available for future rain-splashed infection events.

When cutting out cankers, it is critical to cut past the infection and into healthy wood, in order to stop disease spread. A visual pocket guide on cutting out cankered wood can be found at: sacvalleyorchards.com/prunes/pruners-pocket-guide-for-cutting-out-cytospora/

To help prevent new infections, spraying with protectant fungicides such as Topsin®-M, or Topsin®-M and Rally® WP after pruning and before any rainfall can reduce canker infection.

 

We want to thank the laboratory of Dr. Themis Michailides for their support in diagnosis.