Understanding band canker of almond caused by Botryosphaeriaceae fungi and attempts to control the disease in California

Band canker of almond caused by Botryosphaeriaceae species has been a sporadic disease on 6- to 8-year-old almond trees in California many years after its discovery in the mid-1970s. However, during the mid- to late-2000s, the disease appeared frequently in much younger orchards, 2-4-year-old trees, and especially on trees grafted to vigorous rootstocks (i.e., ‘Hansen’) and in the majority of almond-growing regions. In California, for example, orchards with band canker were recorded in Kern, Fresno, Madera, Merced, Stanislaus, San Joaquin, Sacramento, Yolo, Solano, Colusa, Yuba and Sutter, Butte, Glenn, and Tehama Counties. The aim of this study, financed by the Almond Board of California, was to determine the causal agents of the disease, the period of infection of young almond trees, the spread of the disease among orchards, and develop disease management practices. Seven species were isolated and identified causing cankers on almond trees as follows: Botryosphaeria dothidea, Neofusicoccum parvum, N. mediterraneum, N. nonquaesitum, Diplodia seriata, and Macrophomina phaseolina causing cankers on the trunk of the trees; and B. dothidea, N. mediterraneum, N. parvum, and Dothiorella sarmentorum causing canopy cankers. Inoculations of potted plants of the cultivar ‘Nonpareil’ with B. dothidea periodically for 2 years indicated that the inoculation conducted from March through May caused the largest cankers. The spread of the disease from inoculum sources into the almond orchards was studied in two orchards, one in Fresno Co. with the principal source of inoculum from riparian vegetation along an irrigation canal, and the second in Butte County with the main inoculum source being an old walnut orchard next to the almonds bearing pycnidia and pseudothecia of the pathogens. In both orchards, the spatial pattern showed that the severity and incidence of symptoms declined with distance from the external inoculum sources. However, we have recently detected spatial patterns of band canker severity in very young orchards (2-3 years old) that suggest other inoculum sources, such as early putative infection at nurseries, although this contention needs to be studied in the near future. To manage the disease various methods were attempted. None of the tried methods had significant effect on reducing band canker, except the installation of splitters in the sprinklers in a ‘Nonpareil’/‘Padre’ orchard. Specifically, the size of the band cankers was significantly reduced about 4 months after the alteration of the sprinklers. Also, during the next season, this control practice caused a decrease in both the size and the incidence of the band cankers by 50%. Future research plans on possible endophytic inoculum sources of Botryosphaeriaceae as they transition to become pathogens and new disease management strategies in newly planted orchards will be discussed.