McConnell, M. E., and Balci, Y. 2014. Phytophthora cinnamomi as a contributor to white oak decline in mid-Atlantic United States forests. Plant Dis. 98:319-327.To evaluate Phytophthora cinnamomi as a cause of white oak (Quercus alba) decline in mid-Adantic forests, sampling was conducted at 102 sites from 2011 to 2012. Soil and roots from healthy and declining white oak trees were collected. Phytophthora spp. were isolated using baiting and CFU of P. cinnamomi quantified using wet-sieving. Fine roots were scanned and measured. Phytophthora spp. were isolated from 43% of the sites. P. cinnamomi was common; six other species were isolated infrequently. Little difference in lesion size existed on white oak seedlings inoculated with 32 isolates of P. cinnamomi; only 13 isolates caused significant mortality. Soils from white oak versus nine other hosts did not have significantly different CFU. P. cinnamomi was restricted to United States Department of Agriculture hardiness zones six and seven and never found in zone five. The presence of Phytophthora spp. in soil can be associated with white oak fine root health. When Phytophthora spp. were present, white oak trees in zones five and six had less fine roots. In mid-Atlantic oak forests, however, environmental conditions appear to play a key role in determining the impact of P. cinnamomi on the root system. P. cinnamomi alone does not appear to be a causal factor of white oak decline.

Assessment of the infection status and viability of endophytic Acremonium spp. in USDA plant introductions (PI) of Festuca spp. would aid in identifying potentially desirable endophyte strains for improving pasture and turf species. The PI collection of tall fescue (F. arundinacea Schreb.) and meadow fescue (F. pratensis Huds.) was screened to determine Acremonium infection rate and viability. For tall fescue, 20 seed from each of 565 regenerated seedlots were microscopically examined for endophyte. Seventy‐nine accessions contained endophyte with an average infection frequency of 67.4%. Leaf sheaths of seedlings from the infected accessions were then examined to determine endophyte viability. Fifty‐five of the 79 infected accessions contained viable endophyte at an average infection frequency of 41.2% in the seedlings. The decline in the number of infected accessions and in their infection frequency suggested that loss in endophyte viability may have occurred in storage. More intensive analysis of 15 tall fescue accessions showed that endophyte infection declined from 68.4% in originally collected seed to 42.7% in regenerated seed to 23.6% in the seedlings from the regenerated seedlots, indicating diminishing endophyte occurrence and viability between the time of deposition of seed in the germplasm system and release of regenerated seed for general distribution. In meadow fescue, 29 of 198 accessions contained endophyte at 42.4% infection frequency in the seed and 31.6% infection frequency in the seedlings, indicating a proportionally lesser decline in endophyte viability than in tall fescue. While existing germplasm collections of Festuca spp. can provide Acremonium germplasm for study, newly collected Festuca germplasm will offer a greater diversity of Acremonium genotypes.

Summary The belowground effects of Phytophthora cinnamomi on 1‐year‐old saplings of two common oak species in mid‐Atlantic US forests, white (Quercus alba) and red oak (Q. rubra), were examined after incubation in pathogen‐infested soilless potting mix. Fine root lengths (0–1.5 mm in diameter) of both oak species were quantified after incubation at successive 30‐day intervals up to 300 days, for a total of 10 incubation periods. In addition, colony‐forming units (CFU) of P. cinnamomi were quantified after white oak saplings were incubated in infested soilless potting mix at different temperature/duration combinations that reflect soil conditions present in the mid‐Atlantic United States. Impact of P. cinnamomi on fine root lengths of red and white oak saplings varied considerably over time. Significant periods of fine root loss occurred primarily during spring, when bud break and leaf flush began for both oak species. Red oaks had 17% fine root loss on average, while white oaks appeared more resistant to P. cinnamomi infection with a 2% decrease in fine roots over the course of the experiment. Phytophthora cinnamomi CFU declined significantly with exposure to all incubation temperatures except 8°C. This was in contrast to in vitro experiments, where the optimum temperature for mycelial growth was determined to be 21°C and above. Significant fine root loss caused by P. cinnamomi depended on plant phenology and the oak species tested. Extreme soil temperatures have a significant adverse impact on temporal changes of P. cinnamomi population.

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