Identity and sources of <i>Diaporthe</i> species causing Phomopsis graft dieback disease in macadamia nursery plants

Macadamia is an evergreen tree grown for its edible kernel in tropical and subtropical regions worldwide. The macadamia nursery industry is faced with graft dieback disease, resulting in the rogueing of a large number of grafted plants. These losses are often attributed to Phomopsis graft dieback disease of the clonal root graft (scion) wood. However, the source of scions differs among nurseries, and the identity of the <i>Diaporthe</i> species is unknown. To determine if the same <i>Diaporthe</i> species is responsible for macadamia graft dieback and whether the primary source of infection is from scion wood, we surveyed five major commercial macadamia nurseries in Australia. From 387 macadamia plants sampled in the nurseries, 86% (n = 337 isolates) of the scions were infected with <i>Diaporthe</i> species compared with 14% (n = 53 isolates) of the rootstock. Based on multigene DNA sequence analyses of ITS, TEF, TUB, CAL and HIS, eight <i>Diaporthe</i> species were identified from symptomatic and asymptomatic plants, <i>D. fraxini-angustifoliae</i> was the most dominant species, followed by <i>D. australiana</i>, <i>D. masirevicii</i>, <i>D. litchicola</i>, <i>D. middletonii</i>, <i>D. musigena</i>, <i>D. sojae</i>, and <i>Diaporthe arecae</i>. In the <i>in planta</i> pathogenicity assays, <i>D. litchicola</i> was the most aggressive and caused significantly (P<0.001) larger lesions and more graft dieback than <i>D. australiana</i>, whereas <i>D. middletonii</i> and <i>D. masirevicii</i> produced insignificant lesions, and are therefore considered as non-pathogenic to macadamia. Scion wood inoculation with <i>D. litchicola</i> resulted in more significant Phomopsis graft dieback (45%) than rootstock inoculation (25%). Our findings suggest that scion wood is a potential source of infection and cause of Phomopsis graft dieback in macadamia nurseries.