Localization of a Pathogenicity Gene in Ophiostoma novo-ulmi and Evidence That It May Be Introgressed from O. ulmi

Ophiostoma novo-ulmi, the principal agent of Dutch elm disease, has recently replaced another species of Dutch elm disease pathogen, O. ulmi, across much of the Northern Hemisphere. Field inoculations of the moderately resistant elms Ulmus procera and Ulmus × Commelin were carried out with progeny of a genetic cross between AST27, a Eurasian (EAN) O. novo-ulmi isolate with an unusually low level of pathogenicity, and H327, a highly aggressive EAN isolate. These confirmed the results of a previous study that indicated that the difference in phenotype was controlled by a single nuclear gene. This pathogenicity gene, designated here Pat1, is the first putative pathogenicity gene to be identified in O. novo-ulmi. In a bulked segregant analysis, involving 80 random primers, 10 RAPD (random amplified polymorphic DNA) markers were identified linked to Pat1. Linkage distances between these markers and Pat1 were confirmed by genetic analysis of all individual progeny. Five RAPD amplicons identified in AST27 were O. ulmi and not O. novo-ulmi specific amplicons, and of these two were linked to Pat1. This suggests that the Pat1 allele conferring unusually low aggressiveness in AST27 may have been acquired from O. ulmi via introgression. When RAPD marker OPK31050, linked to Pat1, was used as a probe for Southern hybridization with electrophoretically separated chromosomes of O. novo-ulmi and O. ulmi, the results indicated that Pat1 is located on a 3.5 Mb chromosome previously designated chDNA II.

The Influence of Temperature and Light on Defoliation Levels of Elm by Dutch Elm Disease

The amount of defoliation of elm (Ulmus procera) caused by three Ophiostoma novoulmi Eurasian race isolates over 14 seasons of field trials was found to be strongly correlated with mean air temperature and mean number of sunshine hours over the 12-week period from inoculation to assessment, and with tree age. The coefficient of determination for the regression of percent defoliation on the environmental and tree factors was 0.76, P < 0.001 (33 df). Levels of defoliation were greatest when mean air temperatures exceeded 17°C with moderate light (5 to 7 h of sunshine), and lowest under conditions of either high light (>7.5 h of sunshine) at all air temperatures or low light (<4.5 h of sunshine) and air temperatures of less than 15.5°C. The model varied in its intercept for the three isolates, reflecting their different levels of aggressiveness. The role of environmental factors in the development of Dutch elm disease symptoms and the implications for elm resistance breeding are discussed.

FACTORS INFLUENCING THE MORTALITY OF SCOLYTUS MULTISTRIATUS (COLEOPTERA: SCOLYTIDAE) IN ELM BRANCHES IN CALIFORNIA

Populations of Scolytus multistriatus (Marsham) from two areas of California were sampled in 1979 to investigate various factors influencing mortality. English-elm bolts (Ulmus procera Salisb.) ranging between 2.0 and 12.9 cm in diameter yielded mean mortalities from 49.1 to 86.0%. Mortality was highly correlated with egg density for branch samples from 7.0 to 12.9 cm in diameter. The minimum acceptable elm-branch diameter (2.8 cm) was attacked only during the first sampling period (mid-May to mid-June). Branch diameter, and therefore bark thickness, demonstrated a strong positive relationship with the density of S. multistriatus eggs, attacks, gallery lengths, and emerged adults. Gallery length density was positively correlated with attack density. At low attack densities in small-diameter branches, mean gallery lengths were shorter.