Franz Niederholzer, UCCE Orchards Advisor, Colusa and Sutter-Yuba Counties
Luke Milliron, UCCE Orchards Advisor, Butte, Glenn & Tehama Counties

Canker diseases threaten the profitability of prune growing in California, reducing bearing wood and killing entire trees early in the life of the orchard. These diseases include bacterial canker, Botryosphaeria, Phomopsis, etc. but especially Cytospora. This article is a review of Cytospora canker and a plea (suggestion?) for growers and PCAs to pay close attention to managing what was once largely a nuisance but now is an orchard-threatening problem. We will also review prune orchard management practices that help control Cytospora infections.

Why is the postharvest period so important in the year-round fight against Cytospora?

Cytospora canker is a bark canker caused by one or more species of the Cytospora fungus. Cytospora is a perennial canker; once in the tree, it doesn’t die out in the year of infection. The only way to remove a Cytospora canker from a tree is to cut it out. It is active over a wide range of temperatures (40oF to 90oF), but is most active at higher temperatures (over 80oF). Tree stress is more frequent at higher temperatures (greater risk of water stress and potassium deficiency). Research results show an increased spread of the disease – faster canker growth – with post-harvest water stress.

How does Cytospora infect a tree?

Cytospora is a relatively weak fungus that requires an opening in the tree’s protective bark to infect prune trees in California. (It doesn’t infect flowers.) Such openings include sunburn, bacterial canker damage and pruning wounds. Recent research by Dr. Themis Michailides and his lab has shown that Cytospora can enter prune trees through pruning wounds, something that was tested and found to be only a minor infection site in research from the 1970s. The UC IPM page for Cytospora management in prunes still states that “Pruning cuts… are not important infection sites.” This information is no longer accurate. UC IPM is working to update the entire Prune Pest Management Guidelines.

How does Cytospora spread in an orchard?

Spores oozing from pimple-like structures (pycnidia, see photos in this article) on dead or dying wood are the primary source of new infections. The spores are not released in a dry form to move with the wind but in a starchy strand (cirrus, see figures 1-3) that must be broken up and moved with wind-blown rain. New infections also require wet surfaces from rain or dew or 100% relative humidity for spore germination. Cutting out and burning dead/dying infected wood is a critical management practice for this disease.

What are some possible reasons that may contribute to Cytospora canker becoming such an issue now when it wasn’t several decades ago?

  • New, more aggressive species of Cytospora may now be present in California orchards compared to decades ago.
  • Reduced sanitation via pruning out and burning dead wood due to rising pruning costs and reduced labor availability.
  • Increased orchard stress from reduced inputs and grower attention due to poor prune economics. Warmer, drier fall weather, in general, combined with a possible lack of attention to adequate post-harvest irrigation and resulting water stress may also be a factor.
  • Increased mechanical hedging, creating multiple pruning wounds vulnerable to infection.

How can growers manage prune orchards with Cytospora canker management in mind?

Maintain healthy trees:

    • Maintain adequate potassium nutrition by exceeding crop demand and monitor leaf potassium levels.
    • Avoid water stress throughout the growing season – especially post-harvest – by monitoring soil moisture and/or tree water status (i.e. using a pressure chamber).
    • Avoid sunburn on trunks and scaffolds by painting the bark white, maintaining healthy leaves and tying (roping) branches, when necessary, to prevent the canopy opening too much with crop load.
    • Thin crop load when and where necessary to avoid nutrient stress and canopy opening to sunburn.

Eliminate spore source:

    • Cut out diseased wood several inches below the external canker margin, following this visual guide.
    • Burn dead wood.

Protect pruning wounds from infection:

      • Don’t prune ahead of forecast rain.
      • If you do prune ahead of forecast rain, follow the pruners (or hedger) with a Topsin-M® spray before rain. Rally® can be added to Topsin-M® to reduce resistance development; however, Topsin-M® is the most effective material tested to date.
      • Even if rain isn’t immediately forecasted following pruning, remember that wounds are susceptible to rain-splashed infections for approximately one month. Michailides and his group are doing research this year to test whether immediate treatment after pruning or waiting until just before rain is the best practice.

What can you do during the postharvest period to better manage Cytospora?

  • Maintain stress-free orchards until leaf drop with adequate irrigation.
  • Avoid pruning ahead of rain, especially on young trees where large cuts are near the trunk/crotch area.
  • Cut out Cytospora infected dead wood. Train pruners to cut far enough to completely remove diseased wood. Use this guide or field examples to train pruners to get all the diseased wood out of the trees.
  • Treat pruning wounds with Topsin-M®, with or without Rally ahead of rain.

Figure 1. White Cytospora pycnidia on dead prune wood. These are the spore source for further infections.

Figure 2. From left to right: cut, living bark tissue (A), cut, dead bark (B), and intact, dead bark emitting red cirrus containing Cytospora spores from dark pycnidia (C). Rainwater is needed to dissolve the sugary mucus that holds the spores in the cirrus and wind then moves the spores in rainwater.

Figure 3. Close up of Cytospora cirrus (spore tendrils, see white arrows for examples) emitted from dark Cytospora pycnidia.