Sources of primary inoculum of Tranzschelia discolor in Australian almond orchards

Abstract

Sources of primary inoculum of almond rust (Tranzschelia discolor) were investigated. Retained leaves, fruit not removed during harvest, leaf litter on soil and green or lignified shoots were monitored for viable urediniospores between harvest and the emergence of new leaves. Retained leaves were confirmed as the primary source of overwintering inoculum in almonds with up to 27% of the urediniospores viable at the time of new leaf emergence. Viable urediniospores were also found on symptomless green shoots, retained fruit and lignified shoots. The viability of urediniospores on leaf litter rapidly declined to nil within two months. Inoculation of almond stems and fruit showed that both were capable of producing uredinia, but were less frequently infected compared with leaves. Urediniospores from stem or fruit lesions, symptomless green or lignified shoots, or leaf litter are not considered to play a major role in the epidemiology of almond rust. Inoculation of cherry, peach, nectarine and plum plants with urediniospores collected from almond leaves did not produce disease symptoms indicating that other stone fruit are unlikely to provide an alternative overwintering host for almond rust. Removal of retained leaves in combination with strategic fungicide use may help growers to optimise rust control in almonds.