Rootstock Effects on Almond Leaf Scorch Disease Incidence and Severity

A 5-year field study was conducted to evaluate effects of duration and exclusion of Xylella fastidiosa infections on young almond tree performance and their links to tree vigor. 'Nemaguard,' 'Okinawa,' 'Nonpareil,' and Y119 were used as rootstocks for almond scion 'Sonora.' Among X. fastidiosa-infected trees, there was significant etiological heterogeneity with i) absence of leaf scorching symptoms in the presence of reduced growth, ii) presence of leaf scorching symptoms in the absence of reduced growth, and iii) severe leaf scorching and reduced growth. Trunk cross sectional areas of X. fastidiosa-infected trees grafted on 'Nemaguard' and 'Nonpareil' rootstocks were significantly smaller than noninfected trees, whereas trunk size of trees grafted on 'Okinawa' and Y119 was not affected by infection status. Severity of leaf scorching symptoms was highest on trees grafted on 'Nonpareil' rootstock, intermediate on 'Okinawa' and Y119, and lowest on 'Nemaguard.' X. fastidiosa infections and seasonal leaf scorching symptoms persisted on most inoculated trees throughout the study, except on trees grafted on 'Nemaguard' that manifested complete leaf scorching symptom remission and apparent elimination of the pathogen after the second year. Results indicate that depending on rootstock type X. fastidiosa can affect trunk size in a relatively short period and/or persist for years as trees grow.