How to Protect Your Orchard from Ganoderma

Ganoderma adspersum, was identified in California in 2016 and has become a devastating issue in infected orchards. It's a wood rot fungus that causes decay of the internal structural wood of almond trees. 

Unlike other wood rot fungi, Ganoderma adspersum doesn’t kill almond trees by choking off the vascular system, it rots out the structural wood at the center of the trunk which causes a seemingly healthy tree to suddenly fall over. Because it doesn’t specifically target the vascular system, the trees seem generally healthy until they snap off at the base.  

Ganoderma adspersum versus other wood rot fungi 

Wood rot fungi have been present in orchards for decades but were usually isolated to older orchards. Ganoderma adspersum has the unfortunate ability to infect orchards as young as 4 years old. While the average age of infected orchards is 14 years old; long before growers would plan on redeveloping an orchard.

What Causes Ganoderma adspersum? 

Researchers believe shaking almond trees during harvest allows for entry points for Ganoderma spores. Sweeping the orchards spreads spores into the soil and in the wounds caused by shaking the trees. 

How do I know if my orchard has Ganoderma adspersum?  

While Ganoderma adspersum doesn’t necessarily cause any foliar symptoms before causing trees to fall, it does produce a few signs that you can use to help identify its presence in your orchard.  

Because it is a Basidiomycete, it produces shelf-like mushrooms called conks that can be found on the trunks of infected trees though the presence of conks isn’t required for the disease to be present. Conks vary in appearance but are typically halfcircle or horseshoe shaped, rust red, brown, or white in color, and present on the lower 12 inches of the trunk.  

The presence of these conks usually indicates that the trees have been colonized and the structural wood of the trunk has likely already been compromised. Trees commonly fall over after a long irrigation set or rain event, especially when it is followed by heavy winds.

What causes an infected tree to fall? 

Unfortunately, trees become more prone to falling over when the canopy weight is at its peak which usually corresponds to the point in time where nuts have filled and harvest is close. Trees that fall due to Ganoderma adspersum usually break right at the soil line. It is common to see a fallen tree with very few or no roots attached and white mycelial mats are normally seen on the base of the fallen tree. 

What can you do about it?  

Unfortunately, there aren’t any good control options for Ganoderma adspersum at this point and once you have it in the field, you will likely have trees falling at a steady rate until the orchard is removed. Because this disease isn’t entirely understood, the only management strategies available are general BMPs for good orchard health.  

The one glimmer of hope in the Ganoderma conversation is that infections have been mainly limited to almond rootstock Nemaguard. Viking rootstock was found to be a viable host in an experiment but most infections in the field have been on Nemaguard rootstock.  

Ganoderma adspersum management strategies:  

• Limit bark damage during harvest by managing irrigation and carefully shaking trees  
• Maintain root health to limit wounds and access points for infection  
• Remove infected trees from the orchard to reduce spores in the field  
• Avoid replanting Nemaguard rootstock in a field with the presence of Ganoderma 

The Takeaway 

If you have any questions about Ganoderma or diseases in your field, please consult your Agronomy Sales Enablement Team or your local Simplot Grower Solutions Crop Advisor. 

Authors

Jon Hubble, Northern California
Mike Almasri, Southwest

Click to read more about the authors