Occurrence of Erysiphe buhrii on Gypsophila paniculata in Argentina

Gypsophila paniculata (baby's breath) is a cut flower which is widely grown on the outskirts of La Plata, Buenos Aires province; the main production area in Argentina. In early 1995 a powdery mildew was identified on some greenhouse-grown plants in this area. Since then the disease has occurred each year and has increased in its economic importance. Fungal growth first appears as circular to irregular white colonies, which subsequently develop into an abundant growth on the upper surface of the leaves. The infection spreads to the lower leaf surface, stems and peduncles. In severe attacks basal leaves die prematurely and younger leaves become distorted. The disease occurs throughout the year but mostly in autumn and spring.

The mycelium has well developed lobed appressoria, with condia produced singly on condiophores, with cylindric foot cells. These are 37.5-75 μm (56.6 μm) x 7.5-9.3 (7.8 μm), sometimes flexuous and joined to one or two shorter cells. Cylindrical to ellipsoid conidia were 37.5-52.5 µm (43.3 µm) x 11.2-18.7 µm (14.1 µm). Conidia produced germ tubes that were mostly apically inserted and sometimes with a lobed apex. Based on these features the pathogen was identified as Erysiphe buhrii (Braun, 1995). Ascomata have not been observed to date. A specimen is held in the herbarium of the Instituto Spegazzini de La Plata (LPS 46227).

Both G. paniculata and Dianthus caryophyllus (carnation) are cultivated in La Plata and are hosts of E. buhrii in Europe (Braun, 1995). Detached, mildew-affected leaves of G. paniculata were gently pressed onto and held in place on healthy leaves of both species. These were covered with a plastic bag for 48 h and kept at 22 - 25°C. Tests were repeated with detached healthy leaves under the same conditions. Symptoms similar to those observed on naturally infected plants appeared on G. paniculata at seven to nine days after the treatment, but not on D. caryophyllus. All the control plants remained healthy.

This is the first report of E. buhrii on G. paniculata in Argentina and apparently the first record for South America. Powdery mildew on G. paniculata was recorded as E. communis (= E. buhrii)) in Romania in the 1950s. E. buhrii occurs on various hosts in the Caryophyllaceae in Europe, Asia, North Africa, North America and possibly Central America (Braun, 1995).