Sampling and management of X-disease phytoplasma and vectors

Laura Elizabeth Flandermeyer

doi:10.7273/000006937

X-disease critically impacts stone fruit across the United States, causing fruit to be small, bitter, and poorly colored. X-disease is caused by a phloem-limited pathogen, ‘Candidatus Phytoplasma pruni’ that infects trees, notably sweet cherry (Prunus avium) and other stone fruit, and non-Prunus weedy hosts in groundcover. ‘Ca. P. pruni’ spreads through grafting infected plant material or feeding by leafhopper vectors (Hemiptera: Cicadellidae). X-disease management strategies include tree removal, use of clean plant material during grafting, control of insect vectors, and control of weedy host reservoirs. This thesis focuses on the sampling and management of X-disease phytoplasma and its primary vectors in Washington state, Colladonus montanus reductus (Van Duzee) and Colladonus geminatus (Van Duzee). First, I evaluate three common methods for sampling leafhoppers in orchards (i.e., sticky traps, sweep nets, and vacuum tools) and describe each method, its advantages, and disadvantages to improve decision making. Next, I present the development and analysis of a mathematical model that predicts the acquisition and transmission rate of X-disease in leafhoppers, broadleaf weeds, and trees, and evaluates disease spread under three management strategies: control of weeds, control of immature leafhoppers, or control of adult leafhoppers. In my model simulations I observed the greatest preservation of uninfected trees (i.e., slowest X-disease spread) when insecticides were applied when leafhopper populations first transitioned from nymph to adult. Finally, I sampled 90 cherry trees and broadleaf weeds in the adjacent groundcover in Washington and Oregon from May to October 2023 to understand the distribution of ‘Ca. P. pruni’ in host plants over space and time. I found ‘Ca. P. pruni’ is non-uniformly distributed in cherry trees and visual symptoms were related to titer in wood and leaf tissues on the same limb although titers in leaf tissues were more variable. I observed a correlation between tree titer and the number of infected weeds in the surrounding groundcover. These findings are already being used to improve the monitoring of leafhopper vectors, optimize the chemical control of vectors and weedy hosts, and enhance the sampling of plant tissues for X-disease testing.

Sampling and management of X-disease phytoplasma and vectors

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