First report of powdery mildew on Caragana arborescens in Hungary caused by Erysiphe palczewskii

The Siberian pea tree (Caragana arborescens) is an exotic ornamental shrub with many cultivars. It is a widely cultivated plant in parks in Hungary. In June 2005, during a survey of diseases of ornamental trees and shrubs, powdery mildew infection was observed on Siberian pea trees in one location in Budapest. Later, between June and November 2005, C. arborescens trees in 16 Budapest parks were surveyed. There was only one place where trees were not infected. Over 100 trees were investigated; most of them were either severely or totally infected.

White, epiphytic mycelia and conidia were present on both sides of leaves and young shoots, forming thin, irregular colonies or covering the leaf and shoot surfaces totally (Fig. 1A-C). First developing chasmothecia were observed on 15th June (Fig. 1D). Mass formation of chasmothecia was common on both sides of leaves and infected young shoots (Fig. 1A), during the summer and autumn season. In the case of heavy infection, leaves died and shoots were distorted.

The conidiophores were mostly erect and bore singly hyaline, ellipsoid, barrel-shaped conidia (Fig. 2B & C), which measured 16 x 32 (15-18 x 26-40) µm (pseudoidium-type anamorph). Chasmothecia were subglobose, dark brown to black, measuring 143 (90-160) µm in diameter, with irregularly polygonal cells (Fig. 2A & D), equatorial appendages, 9-12 per chasmothecium (Fig. 2A), which were straight, hyaline, smooth, dichotomously branched, with some morphological variation of the apex (Fig. 3) and measuring 253 (230-312) µm. The asci were shortly stalked, 6-10 per chasmothecium, 4, 6 or 8 (sometimes 2) spored (Fig. 2E & F), measuring 29 x 69 (26-33 x 60-75) µm. The ascospores were ellipsoid-ovoid, measuring 15 x 26 (10-18 x 20-28) µm.

Based on this morphology the identification of this powdery mildew fungus was confirmed as Erysiphe palczewskii (syn. Microsphaera palczewskii; Braun & Takamatsu, 2000), by comparison with data given by Gelyuta & Gorlenko (1984) and Gelyuta & Minter (1998). Specimens are preserved in the Herbarium of the Plant Protection Institute at Budapest (accession number: Mp 1-5/2005).

This fungus is an introduced Asian species, which is mainly known to occur on Caragana spp., and has been recorded in recent years in several many European countries (Gelyuta & Minter, 1998), and in several states of USA (Idaho, Washington, North Dakota, Minnesota and Alaska). This is the first report of Erysiphe palczewskii in Hungary.