Susceptibility of Garden Plants to Phytophthora Root Rot.

Phytophthora root rot (PRR) is a serious disease of horticultural, forest, and ornamental plant species caused by species of the oomycete genus Phytophthora. Their wide host range makes the choice of resistant plants in the management of the disease difficult. We used the Royal Horticultural Society diagnostic dataset of PRR records from U.K. gardens to compare the susceptibility of different host genera to the disease. The dataset was compared with existing reports of plants recorded as notably resistant or notably susceptible to PRR. An index-based approach was used to separate 177 genera of woody plants into three categories: 85 were low index (<0.10: rarely affected), 34 were medium index (0.10 to 0.20: sometimes affected) and 58 were high index (>0.20: frequently affected). Similarly, genera of nonwoody plants were separated into: 45 low index (<0.22), 16 medium index (0.22 to 0.44), and 18 high index (>0.44). Taxus was the genus with the highest index, while most genera in the Malvales and Ericales were in the high-index group. Most genera in the Myrtales, Fabales, and Monocotyledons were low index. While 30 Phytophthora species were recorded in our study, the wide host range spp., P. plurivora, P. cryptogea, and P. cinnamomi, represented 63% of identifications. P. plurivora was the most common species on woody plants and P. cryptogea on nonwoody plants. These results provide confidence in the use of host resistance as part of the integrated management of PRR.

Susceptibility of Garden Trees and Shrubs to Armillaria Root Rot.

Armillaria root rot (ARR) is a serious disease of woody plants caused by several species of Armillaria. Armillaria isolates from diagnostic samples received in 2017 were identified by genus- and species-specific PCR and compared with isolates from an earlier survey (2004 to 2007). The results were comparable and, therefore, were combined for further analysis. Three species were identified: Armillaria mellea (83%), A. gallica (15%), and A. ostoyae (2%). Their wide host range makes choice of resistant plants in management of the disease difficult. We used the Royal Horticultural Society diagnostic dataset of ARR records from U.K. gardens to compare the susceptibility of different host genera to the disease. The dataset was compared with an earlier experiment at the University of California. An index-based approach was used to separate genera into three categories: 77 low-index (<0.99), 37 medium-index (0.99 to 1.76), and 56 high-index (>1.76) genera were recorded. All three species were associated with both angiosperms and gymnosperms; moreover, A. ostoyae did not show the host preference for gymnosperms that has been reported elsewhere. A. gallica was particularly common on herbaceous perennials and showed a trend to occur on resistant hosts that may be under other stress, supporting its description as an opportunistic pathogen. Four monocotyledons grown as trees or shrubs in U.K. gardens had a very low ARR index according to indices associated with A. mellea and A. ostoyae. Genera in the order Myrtales were almost always low index, while those in the Saxifragales and Fagales were mostly high index. These results provide confidence in the use of host resistance as part of the integrated management of ARR.