First report of walnut canker caused by Fusarium incarnatum in Argentina

The agronomic practices of walnut (Juglans regia L.) production in Argentina have improved significantly over the last 10 years. New walnut varieties have reduced the average cropping period from 12 years to 7, in addition to improving the overall economic value of the crop, through greater yield and improved fruit quality. In 1999, disease symptoms were observed on 1 year old grafted-walnuts from the Zavalla locality in Santa Fe province. Primary symptoms were cankers of varying sizes (up to 50 mm) principally on the main stem but also on side branches. On walnut varieties Chandler, Davis and Tulare, cankers were seen in lateral and terminal buds. Fruit symptoms consisted of necrotic brown spots, that were 20 mm in diameter and 5 mm deep. Affected fruit would abscise prematurely. In 2000 and 2001, the same symptoms were detected in a walnut collection of the Facultad de Ciencias Agrarias of Rosario University.

Necrotic tissue fragments taken from diseased plants were surface-sterilised and plated on potato dextrose agar. A Fusarium species was consistently isolated into pure culture. Single spore cultures from these colonies were grown on carnation leaf agar to assist species identification (Nelson et al., 1983). The fungus was identified as F. incarnatum (Rob.) Sacc. (=F. pallidoroseum (Cooke) Sacc = F. semitectum Berk & Rav.) based on its micro-morphology and cultural features. For pathogenicity test, a conidial suspension (1 x 10⁶ conidia per ml) was sprayed onto wounded branches of 1 year old walnut cv Tulare plants. Control plants were sprayed with water. The inoculated plants and controls were placed in a humid chamber at 20-25°C and 85% RH for 2 days. The plants were observed daily for symptom development. All the inoculated plants showed dieback at day 30 and cankers appeared 60 days post inoculation. None of the wounded control plants developed any symptoms. F. incarnatum was successfully reisolated from all the inoculated plants, thus fulfilling Koch's postulates.

This fungus has previously been recorded affecting walnut in Italy (Belisario et al., 1999). This is however, the first report of walnut canker caused by Fusarium incarnatum in Argentina.