Pythium myriotylum, a new pathogen of green beans in Spain

In 2001 and 2002, bean plants (Phaseolus vulgaris 'Festival' and 'Donna') cultivated on rockwool slabs in two commercial greenhouses in southeastern Spain were observed with root necrosis and necrotic streaks at the base of stems. In one greenhouse a few, scattered plants were affected, while in the other we estimated 20-30% dead plants occurring in distinct clusters. We consistently isolated a Pythium sp. from roots and stems of the affected plants on potato dextrose agar and cornmeal agar amended with pimaricin, ampicillin and rifampicin (P5ARP5).

Single hyphal transfers gave pure cultures of this oomycete. Four isolates from different plants produced filamentous inflated sporangia with zoospores and aplerotic oospores. Sequences of ribosomal DNA from ITS1 region, 5.8S rDNA and ITS2 were identical for all four isolates and that from isolate Py-V3 has been deposited as GenBank Accession No. AM396958. The isolates were identified as P. myriotylum on the basis of molecular sequences and morphological characteristics (Van der Plaats-Niterink, 1981).

We inoculated P. vulgaris 'Emerite' at the 5-7 leaf stage with three isolates (Py-V1, 2 and 3) by irrigating plants with a 50ml of mycelial suspension per plant, obtained by growing isolates on potato carrot agar in 9 cm Petri dishes (PCA) until they fully covered the surface, then blending and homogenizing mycelial growth in 300 ml sterile distilled water. The plants were grown in plastic bags filled with perlite and kept in a greenhouse where temperatures varied from 22-31°C. After 29 days all plants had necrotic roots, some with stem streaks and some eventually died, similar to the disease development originally observed. Control plants irrigated with sterile distilled water washed off uninoculated PCA remained healthy. When we repeated the inoculations at a lower temperature range (13-30°C), isolates Py-290 and Py-V2 caused root necrosis in all inoculated plants but stem streaks and death in only a few. P. myriotylum was reisolated from inoculated plants. High temperatures are associated with more severe disease development (Gay, 1969).

To our knowledge, this is the first report of P. myriotylum as the causal agent of root and stem necrosis of adult bean plants in Spain.