Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/27/2016
Publication Date: 5/3/2016
Citation: Krugner, R., Ledbetter, C.A. 2016. Rootstock effects on almond leaf scorch disease incidence and severity. Plant Disease. 100(8):1617-1621. doi: 10.1094/PDIS-01-16-0125-RE.
Interpretive Summary: Almond leaf scorch disease (ALSD) is caused by the bacterium Xylella fastidiosa, which is transmitted by sharpshooters and spittlebugs. The vast majority of the 380,000 ha of almond orchards in California is located in the San Joaquin Valley, where significant heterogeneity of disease incidence and severity exists among regions and cultivars affected by ALSD. Although cultivar selection and planting location partially describe the heterogeneity in ALSD intensity during the history of almond production in California, a clear alternative has not been proposed. A five-year field study was conducted to evaluate effects of duration and exclusion of X. 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’. Symptoms of leaf scorching, reduced growth, and X. fastidiosa infection persisted throughout the study on all trees grafted on ‘Nonpareil’ rootstock, whereas remission of leaf scorching symptoms and elimination of the pathogen occurred in only 30% of the X. fastidiosa-inoculated trees on ‘Okinawa’ and Y119 rootstocks. ‘Nemaguard’ promoted complete pathogen elimination and remission of leaf scorching symptoms. Results indicate that a X. fastidiosa-resistant trait in the rootstock can be valuable for maintaining low incidence of ALSD in California.
Technical Abstract: A five-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 1) absence of leaf scorching symptoms in the presence of reduced growth, 2) presence of leaf scorching symptoms in the absence of reduced growth, and 3) 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 non-infected 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’. Xylella 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 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, potentially serving as source of inoculum for secondary pathogen spread.