Effect of initial darkness duration on the pathogenicity of Calonectria pseudonaviculata on boxwood

Abstract

Light is an important environmental stimulus that regulates many physiological activities within plants and pathogenic fungi. Boxwood blight (causal agent: Calonectria pseudonaviculata) is a newly emergent disease in the United States and a significant threat to the boxwood industry. A disease-forecasting model has been developed to aid boxwood growers in their management decisions, but light is not included as a variable within the programme. Growth chamber experiments were conducted to evaluate the effect of an initial dark period on disease severity, mycelial growth and conidial germination of C. pseudonaviculata. Treatments varied by darkness duration (0, 6, 12 or 24 h dark), and were followed by a 14 h day length at a 25/21°C day/night temperature. Neither boxwood blight severity nor fungal colony development was enhanced by an initial dark period. However, darkness duration was associated with a significant increase in conidial germination in vitro. This study suggests the predictive performance of the current boxwood blight disease-forecasting model would not be improved by inclusion of photoperiod.

Boxwood Blight: Threat to Ornamentals

“…Shaded plots saw significantly more severe disease than full-sun plots in a Virginia study (63). Marine et al (69) tested the effect of the length of the daily darkness period on disease development but found no effect on boxwood blight severity.…”

Boxwood blight: an ongoing threat to ornamental and native boxwood

“…insularis ‘Nana’, and B. microphylla ‘John Baldwin’ show no symptom development at inoculum doses below 5000 spores/mL, but it is not known whether latent infection occurred under these conditions (Avenot et al 2017 ). Germination of conidia is increased when they are exposed to 24 h of darkness following a 14-h photoperiod, compared to germination rates when conidia are exposed to 12 or 24 h of light following the same 14-h photoperiod (Marine et al 2017 ). This is consistent with findings that show numerical—but non-significant—increases in disease severity on shoots of susceptible cultivars B. sempervirens ‘Suffruticosa’ and B. sempervirens ‘Justin Brouwers’ exposed to 12 or 6 h of dark after inoculation by C. pseudonaviculata conidia (Marine et al 2017 ).…”

“…Germination of conidia is increased when they are exposed to 24 h of darkness following a 14-h photoperiod, compared to germination rates when conidia are exposed to 12 or 24 h of light following the same 14-h photoperiod (Marine et al 2017 ). This is consistent with findings that show numerical—but non-significant—increases in disease severity on shoots of susceptible cultivars B. sempervirens ‘Suffruticosa’ and B. sempervirens ‘Justin Brouwers’ exposed to 12 or 6 h of dark after inoculation by C. pseudonaviculata conidia (Marine et al 2017 ). Together, these findings illustrate the need to either prune out infested branches or completely remove diseased plants from the nursery or landscape, to reduce the potential inoculum load in the environment, and to increase light penetration into the canopy to reduce the pathogen’s ability to initiate fresh infections.…”

“…Humidity also plays a key role in the infection process and pathogen growth and survival. When tested in vitro, mycelial growth, as measured by colony size, was increased at 65% RH compared to 95% RH, but it is unknown from these experiments whether any coincident changes in conidial production and/or microsclerotium formation occurred (Marine et al 2017 ). In a separate study, Avenot et al ( 2017 ) showed that by interrupting wet conditions (100% RH) for 3 h or more with a dry period (RH < 65%) at 8 h post-inoculation, a significant reduction of disease symptoms occurs.…”