First Report of Biscogniauxia mediterranea on Quercus emoryi in Southern Arizona.

Abstract

In 2019, a decline of Quercus emoryi (Emory oak) was observed on the Coronado National Forest located in southeastern Arizona. Symptoms associated with oak mortality included crown die-back and large cankers revealing charcoal-like stromal growth. Trunks and root collars showed girdling and many affected trees also displayed evidence of gold-spotted oak borer activity. Initial surveys in stands identified clusters of severe infections. Samples with black perithecia and stromal tissue were collected from symptomatic hosts. Morphological characterization of the fungus was completed on fresh perithecial tissue. Stromata were pulvinate and black showing embedded perithecial bumps, with ostioles visible from the surface of the stroma. Asci were short stipitate and cylindrical with visible oil drops, 6.6 to 9.4 (mean: 8.8) × 139.8 to 179.9 μm (mean: 166.4). Ascospores were smooth ovoid, brown to dark brown, with narrowed and round ends, 6.9 to 9.1 (mean: 7.7) × 13.8 to 25.9 μm (mean: 16.5). Colonies grown on ½ strength potato dextrose agar (½ PDA) (Korhonen 1980) at 25°C for 14 days were whitish-grey when viewed from the top and darkened embedded spheres were visible from the bottom. Single-spored cultures were isolated by dissecting the hymenium and placing in distilled water. The suspension was streaked onto ½ PDA plates and incubated for 12 hours, and pure cultures were grown on ½ PDA. DNA was isolated from a mycelial scrape and extracted using a 10% Chelex solution (Safaee 2017). The internal transcribed spacer region was amplified with the ITS1/ITS4 primers (White et al. 1990) and PCR products were sequenced at Eurofins Scientific (Lewisville, KY). Isolates (BSS1, BSS2, BSS3, BSS4) were compared to other Biscogniauxia mediterranea sequences in the NCBI database (MG098274, EF026134) and had 100% sequence identity. Pathogenicity assays were conducted using 8-year-old Emory oaks in a greenhouse. The oaks were drought stressed and watered at 50% pot capacity for two months prior to inoculation. Each tree received approximately 2400 ml of water every two weeks for the remainder of the experiment. Eight trees were wounded three times with a 5mm corer. A 5mm plug of mycelium of isolate BSS2 was used to inoculate wounds. Four trees were used as controls and inoculated with ½ PDA plugs. Wounded and inoculated areas were assessed after three months. All eight trees exhibited dark necrotic tissue in the vascular cambium and lesions ranged from 11.4 - 25.8 mm in length and were 3.9 - 13.3 mm wide. The controls exhibited no fungal sign, lesions, or necrotic tissues. The pathogen B. mediterranea was recovered from 13 of the 24 total inoculations and its identity was confirmed with ITS sequencing. Mortality and die-back caused by Biscogniauxia spp. are commonly associated with drought-stressed trees, with increasingly hot and dry conditions routinely noted as inciting factors (Desprez Loustau et al.). Emory oaks are among the dominant tree species in much of the Madrean oak woodlands of the southwestern US and Mexico and provide vital ecological and cultural services. As southwestern states continue to experience hotter and drier, conditions, it is likely Emory oak will become increasingly susceptible to die-back and mortality due to this Biscogniauxia species (Southern et al.).