In Oregon, cool to warm (59 to 77☏) wet conditions are favorable for disease development and high humidity levels or free water is needed for infection to occur. The fungus penetrates the leaf through stomata and wounds. The fungus produces long-lived survival structures called microsclerotia within infected tissue, which are released into the soil from plant debris, and can initiate new infections by producing infective spores or by being splashed onto foliage directly. Mild wet weather west of the Cascade Range is conducive for long-term survival and infection. The fungus can produce spores in a sticky, slimy matrix when environmental conditions become suitable. The fungus can survive in fallen leaves for up to 5 years but only 3 years or less in garden soils. sempervirens), 'Dee Runk', 'Green Gem', 'Green Mountain', 'Green Velvet', 'Suffruticosa', and 'Winter Gem'. In Oregon nurseries, the fungus was isolated from 46 different cultivars and was frequently found on common boxwood ( B. Severe disease may also develop on any cultivar when canopies remain wet for extended periods of time. For example, 'Winter Gem' is fairly resistant to boxwood blight at 77 ☏, but develops severe disease at cooler temperatures ranging from 59 to 68☏. japonica 'Green Beauty' are more resistant to disease, although they still carry the fungus and severe disease can still develop if environmental conditions are conducive. insularis (Korean boxwood)-are known hosts as well as B. The three commonly grown boxwood species- Buxus sempervirens, B. and cultivars, Pachysandra, and Sarcococca. Due to the history of this disease, its explosive life cycle, and subtle symptoms on tolerant plants, it is expected to be an ever-increasing problem throughout the PNW. Plants that are tightly spaced in flats, grown pot-to-pot, or other spacing where plant canopies touch each other are also at risk for more severe disease. This includes plants in propagation greenhouses, those watered too frequently without sufficient time to dry between irrigation events, or are shaded by taller plants or structures. In nurseries, boxwood blight is severe in situations where there is high humidity or where moisture remains on leaves for long periods of time. ![]() The disease is especially severe in the landscape where plants are sheared tightly and/or planted in hedges. Calonectria henricotiae also causes boxwood blight but is found only in Europe and is resistant to many fungicides. It has been a problem for many years in Europe and New Zealand. pseudonaviculata is mainly of one major DNA type and is significantly differentiated from the population found in Europe. The disease has been found in 30 states including the District of Columbia, at least 16 Oregon nurseries (in Clackamas, Linn, Marion, Multnomah, Washington, and Yamhill counties) and also in landscapes in Clackamas, Coos, Lane, Multnomah, and Yamhill counties and in Seattle, WA. buxicola) was found for the first time late in 2011 in three Oregon and four British Columbia nurseries. Cause Boxwood blight caused by the fungus Calonectria pseudonaviculata (synonyms Cylindrocladium pseudonaviculatum or C.
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