Jaime Ott, UCCE Tehama, Glenn, Shasta, Butte
Luke Milliron, UCCE Butte, Glenn, Tehama

Infection by Cytospora and other fungi is a major cause of scaffold and tree loss in prunes, resulting in reduced yield. These fungi can also infect young trees, significantly reducing the return on investment of an orchard. We have written extensively about the tremendous value in immediately protecting pruning wounds with Topsin-M* to reduce these canker diseases. In addition, we are now learning that this fungicide can reduce canker infections beyond protecting pruning wounds.

Bottom line

• Tree stress and injury (including pruning/hedging wounds) are the two major factors triggering Cytospora disease
• In young trees, a March fungicide spray on the trunk and primary scaffolds reduced incidence of cankers
• Protecting pruning or hedging wounds with Tospin-M significantly reduces the incidence and severity of disease
• Pruning/hedging is best done after harvest when the weather is dry and there are limited Cytospora spores in the orchard
• Employing the practices above is particularly important from February through May, the highest risk period for new infections

*mention of Topsin-M is not a pesticide recommendation, simply the sharing of research results. Consult your PCA and always read the pesticide label; the label is law. If considering two Topsin-M fungicide applications per year make sure to avoid exceeding the lbs active ingredient (AI/ac) limit on the pesticide label.

Cytospora and other related fungi are common pathogens found in California prune orchards. Infection can cause the loss of major scaffolds, or even entire trees, which can lead to reduced orchard lifespan. Symptoms of Cytospora infection include gumming and a slightly sunken area under the bark where there is dead, infected tissue (called a canker). These cankers are frequently seen extending down scaffolds from a pruning cut or a sunburnt area of the branch. 

Cytospora in young orchards and replants
In young orchards, Cytospora infections are particularly damaging. Generally, cankers are found on the trunk and primary scaffolds and frequently result in tree death. Previously, we thought that these cankers were untreatable. However, recent research by Themis Michailides at UC Davis has shown that a Topsin spray in late March to the trunk and crotch of young trees (Figure 1) can reduce the percent of trees showing canker symptoms in the fall. 

Cytospora from tree stress
Other work by Themis Michailides has shown that Cytospora can cause latent infections in trees. Latent infections are when a pathogen has colonized (infected) the tree tissue but has not caused visible disease symptoms. The latent infection can be triggered into an active, disease-causing infection when the tree is subjected to stress, such as sunburn, potassium deficiency, freeze damage or drought stress. An example of a latent infection switching to an active infection in humans are cold sores, caused by the herpes simplex virus.

To study latent infections in prune trees, the Michailides lab did an experiment where they made pruning wounds, wrapping half of those wounds immediately after making the cut to protect them from infection and leaving the other half unwrapped. The unwrapped wounds showed a high level of infection, which would be expected from external inoculum landing on the cut surface. However, even 10-20% of the wrapped wounds developed disease. This indicates that those branches had a latent infection, with the Cytospora present in the tree but not causing symptoms. When the pruning cuts were made, this stress was enough to trigger the Cytospora to cause visible disease in these branches. 

The major key to managing disease caused by latent infections is to reduce the amount of stress the trees are under. Adequate irrigation, adequate potassium, and reasonable cropload are important to reducing tree stress and are under your control as a grower. 

Cytospora on pruning/hedging wounds
Cytospora and other related fungi are known to infect through injuries, including the wounds caused by pruning and hedging. In this case, fungal spores are carried by splashing water (from rain or sprinklers) onto fresh cuts or injuries, where they germinate and grow into the living plant tissue. In some orchards, this can happen in nearly 100% of pruning cuts, depending on the amount of inoculum (spores) at the time of pruning. Management of these kinds of infections boils down to:

1.  Reducing the amount of inoculum in the orchard. Cytospora lives and produces spores on pruning brush, dead branches, and dead trees. Removing diseased branches, dead trees, and prunings from the orchard and burning them, rather than shredding, can reduce the amount of inoculum. Cytospora can survive for over a year on shredded prunings on the orchard floor.
2.  Pruning/hedging when there is little to no chance of rain splashing spores onto cuts. Cytospora can cause infection for 30 days after pruning but is most severe if there is rain in the 15 days after pruning. Although we can’t reliably predict weather 2+ weeks in advance, you might reconsider pruning if there is a chance of rain in the 10-day forecast. You can also consider pruning/hedging right after harvest when the weather is dry. Research has shown that spore levels in orchards are at the lowest at the end of the summer, before any fall rains.
3.  Protecting the pruning/hedging cuts from infection using Topsin-M. Topsin-M both reduces the incidence and severity of Cytospora infections in prunes over many years of research. This is NOT a silver bullet: it must be applied before the fungal spores land on the pruning cut, and it does not reduce disease incidence to zero. However, used along with the other management strategies listed above, Topsin-M is our best option for preventing Cytospora infections of pruning/hedging wounds. 
4.  As we will remind you in summer – once Cytospora cankers are visible, the only way to control the disease is to prune out infected branches. Cut into healthy wood well past the visible canker. Prune out diseased branches after harvest when it is dry. You can dip pruning tools into a 1:9 bleach: water solution to help avoid spreading the disease. 

Michailides Lab research tracking fungal spores and latent infections shows that February through May is the highest risk period for developing new infections. During this period, you can reduce your chance for disease by avoiding pruning and by spraying young trunks and tree crotches with Topsin-M.