Tree decline linked to fungi, pests

The graft union is a critical juncture when it comes to Sudden Apple Death (SAD) a phenomenon that affects young apple trees across the Okanagan.

Staff from the Summerland Research and Development Centre delivered a SAD update on July 13, noting that it presents as a combination of factors whose effects may be accelerated by climate, soil conditions and growing systems.

The primary factor in SAD is Cytospora, a fungal pathogen that impact apple trees at the graft union, says Jesse MacDonald, the knowledge and technology transfer specialist at Summerland.

“We are finding that [it] forms cankers at the graft union,” he says.“It is considered a mild pathogen, but in recent years it has been playing a more important role in orchard health in the Okanagan.”

The second major factor is apple clearwing moth, an invasive species that is reaching critical mass in most orchards in the Okanagan and also attacks the graft union.

“The arrival of ACW corresponds with SAD occurrence,” says MacDonald.

The combination of fungal and insect attacks is reduced water transport through the graft union, indicated by clear signs of water stress.

The situation is exacerbated by a greater number of days each year with extreme heat.

“We are getting more 35° C days and the trees need more water than they have in the past,” says MacDonald.

Under those conditions, well-drained soils that lack good water-holding capacity become a liability, according to Kirsten Hannam, a systems agro-ecologist examining water, carbon and nutrient dynamics in soils.

“Shallow, coarse-textured soils that lack organic matter do not retain water,” notes Hannam.

The high-density growing systems that have been popular with growers may be a liability, too.

“I’ve looked at older trees with bigger trunks that have huge cankers and huge ACM damage and they show less impact,” notes MacDonald.

MacDonald says the research is continuing, with potential solutions including more robust rootstocks.

The lecture was part of a new series of extension offerings that reflects the work of the province’s tree fruit industry stabilization initiative.

The series is organized in conjunction with the BC Institute of Agrologists and spearheaded by Adrian Arts, the province’s tree fruit and grape specialist and owner of Kamla Orchards in Summerland.