Epidemiology of Alternaria blight of Paulownia

Paulownia trees are indigenous to China, where they are found in all regions from the sub-tropical south, to the plains and mountainous areas in the north. In Australia, a number of Paulownia plantations have been established in recent years as the timber is a valuable resource with strong export potential. The main species grown are P. fortunei and P. tomentosa. In Western Australia, major plantations have been established to the north of Perth, in a Mediterranean environment with irrigation and fertigation over the summer months. At some sites the trees are subject to strong winds that can cause significant physical damage to the large leaves, and sunscald also has a major impact on tree growth. In addition to abiotic factors affecting the health of the trees, foliar diseases are also present. Alternaria blight (1) and a phytoplasma-associated disease (2) have been reported, and other pathogens are regularly being identified. Here we outline some of the major findings from research being conducted on Alternaria blight of Paulownia in Western Australia

Temperature, humidity, wounding and leaf age influence the development of Alternaria alternata lesions on leaves of Paulownia fortunei

Alternaria blight, caused by Alternaria alternata, is a recent and serious disease of commercially grown Paulownia trees in Western Australia. This study investigated the effect of temperature, humidity, wounding and leaf age on lesion development. Temperature had a significant effect on infection, with lesion development and expansion observed to increase from 15-25°C, declining between 30 and 37°C. Relative humidity (RH) also influenced infection with limited lesion development observed at ≤92% RH whereas incubation at 98 and 100% RH resulted in large lesions. The optimum conditions determined for lesion development were incubation at 25-30°C with 98-100% RH. Wounded leaf tissue was significantly more susceptible to infection than non-wounded leaf tissue and senescent leaves were more susceptible than newly emerged leaves. Variation in pathogenicity was also observed on senescent leaves infected with different pathotypes of A. alternata. These results appear to correlate well with field observations and may be useful in the control, management and forecast strategies for Alternaria blight in Australian Paulownia plantations.