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Sustained water stress increases black walnut susceptibility to the pathogen Geosmithia morbida

Rachael A. Sitz: Department of Bioagricultural Sciences and Pest Management, Colorado State Univeristy

<div><em>Geosmithia morbida </em>is the fungal causal agent of thousand cankers disease (TCD) of walnut, and black walnut (<em>Juglans nigra</em>) is particularly susceptible. This disease has spread throughout the western states, as well as several eastern states, though disease may be more severe in the west due to the increased drought stress from the arid climates. However, neither black walnut response to drought, nor tree susceptibility to TCD under drought conditions is well understood. In this study, we drought stressed black walnuts by maintaining them at 60% pot capacity, and compared their drought and infection responses to well-watered controls which were maintained at 100% pot capacity. Physiological measurements including mid-day water potential, stomatal conductance, and photosynthesis were conducted. To quantify canker sizes over time (12 hour, 24 hour, 2 day, 4 day, 8 day, and 16 day measurements), trees were inoculated with <em>G. morbida </em>isolate 1217 using a 5 mm core punch and the bark was removed to calculate the lesion area. Overall, cankers were larger on trees in the drought stressed treatment, and increased over time (<em>P</em>>0.05). Black walnut trees displayed isohydric behavior, as drought stressed trees sustained similar mid-day water potentials when compared to the well-watered treatment (<em>P</em>>0.05). However, photosynthesis and stomatal conductance decreased in drought stressed trees (<em>P</em>>0.05 for both measurements). Taken together, these results suggests that <em>J. nigra</em> compensates for drought stress by using an isohydric behavior that conserves water through limiting stomatal conductance. This strategy may compromise its ability to defend against the canker pathogen <em>G. morbida</em>.</div>