A bio-assay to screen Ilex crenata for tolerance to high soil pH and black root rot resistance

Ilex crenata or box-leaved holly has become a popular alternative to boxwood (Buxus spp.) as a result of the plants’ similar appearance. However, box-leaved holly prefers acidic soils whereas boxwood prefers alkaline soils, which are more prevalent in private gardens. When grown in alkaline soils, box-leaved holly exhibits problems in nutrient uptake and can become infected with black root rot caused by Berkeleyomyces basicola, a soil-borne fungus that is favoured in alkaline conditions. Breeding box-leaved holly for tolerance to alkaline soils and resistance to B. basicola can solve these issues. So far, Ilex crenata cultivars mostly are the result of chance findings. Recently, targeted breeding to improve (a)biotic stress tolerance has gained interest. To screen potential breeding parents and seedlings for tolerance to high soil pH and black root rot, a bio-assay is needed. Two trials were conducted to develop a protocol for this bio-assay. One-year-old plants or rooted cuttings were grown in soil of pH 4, pH 5.5 or pH 7 and inoculated with B. basicola or treated with water (negative control). In one trial, three B. basicola isolates were tested. Disease symptoms and plant stress (measured by chlorophyll fluorescence) were evaluated after 10 or 13 weeks. Especially the combination of a high soil pH and inoculation with B. basicola caused severe discoloration of the roots and growth inhibition of the shoots. Application of only one stress factor led to less severe symptoms. A protocol for a bio-assay was derived from this study.