Contribution of Fallen Leaves to Primary Inoculum and the Progress of Glomerella Leaf Spot in Apple Orchards

ABSTRACT

Glomerella leaf spot (GLS), caused by Colletotrichum spp., persists between seasons mainly in fallen leaves on the orchard floor and in dormant plant tissues, which act as primary inoculum sources for disease initiation in spring. Because early epidemic development is strongly dependent on the availability of primary inoculum, practices such as the removal of fallen leaves could delay symptom onset and contribute to reducing disease progress. This study evaluated whether reducing primary inoculum originating from fallen apple leaves on the orchard floor affects the onset and progress of GLS. Two field experiments were conducted during the 2020/21 (Exp1) and 2021/22 (Exp2) seasons, comparing treatments with and without the removal of symptomatic leaves fallen on the ground during the dormant period. Overall, the presence or removal of fallen leaves did not influence the incidence of latent infections in leaves nor reduce the area under the disease progress curve. In Exp2, the number of conidia captured by spore traps was higher in traps positioned at 1.80 m above the ground, regardless of the presence of fallen leaves on the orchard floor. These results indicate that the management of fallen leaves alone is insufficient to substantially reduce the primary inoculum of Colletotrichum . This finding is particularly relevant in orchards with a history of GLS epidemics, where inoculum is likely already established in perennial plant tissues. Therefore, additional and integrated management strategies are required to effectively reduce disease progress driven by primary inoculum.