Susceptibility screening of various Buxus cultivars to Cylindrocladium buxicola

Cylindrocladium buxicola (synonym C. pseudonaviculata) is an ascomycete fungus which causes box blight in Buxus species. Under warm and wet conditions, disease symptoms can increase rapidly, causing substantial economic losses to the boxwood sector. First detected in the UK in the mid-
1990-s, the disease has spread throughout Western Europe. At the end of 2011, C. buxicola was found in the USA, increasing the impact of the disease. Another concern is the recent identification of a second genotype of C. buxicola in Europe.
As part of integrated disease management, we tested the sensitivity of 33 different Buxus species and cultivars to the two known genotypes of C. buxicola. No differences in pathogenicity were noticed between the two genotypes or between different isolates of the main genotype. Consequently, one isolate was selected for future research. The main tests were conducted under semi-commercial conditions, including the effect of sporulation and secondary spread. We also investigated how temperature and duration of leaf wetness affected the infection capacity of C. buxicola. Healthy plants of the different species and cultivars were placed outdoors, around a central source of inoculum, artificially inoculated plants of the very susceptible Buxus sempervirens ‘Suffruticosa’. Disease development was assessed on a weekly basis. Temperature, relative humidity, rainfall and wind speed were logged and interpreted in terms of conduciveness to disease development.
No resistant Buxus cultivars were detected. Under controlled optimal environmental climate conditions for the disease, all cultivars became affected to some extent. Young leaves of all cultivars were very susceptible, but the mature leaves of some cultivars (such as B. microphylla ‘Faulkner’) exhibited markedly reduced susceptibility under conducive weather conditions for the disease. Very susceptible cultivars (such as B. sempervirens ‘Suffruticosa’) already developed extensive disease symptoms under such conditions.
To better understand how temperature and leaf wetness period affect the infection capacity of C. buxicola, inoculation experiments were conducted under controlled environmental conditions. The minimum leaf wetness period for infection was cultivar-dependent. Within each cultivar, this period was also considerably shorter for young leaves compared to mature leaves. In addition, young leaves could be infected at lower temperatures (6-12 °C) than mature leaves (12-17 °C).
Based on these results, we believe that both leaf maturation level and temperature affect the minimum leaf wetness period needed for infection. This can ¬¬- partially - explain the observed weather-dependent susceptibility level of different Buxus cultivars.
In the short term, this research will contribute to a better risk assessment of the different Buxus cultivars. In the long term, it can contribute to the development of a predictive risk model, based on cultivar and weather data.