First Croatian report of powdery mildew on tomato caused by Oidium neolycopersici

During 2008 powdery mildew symptoms were observed on tomato plants (Solanum lycopersicum) grown in a greenhouse in north-western Croatia. Whitish rounded spots of superficial mycelium were present on the upper surfaces of the leaves (Fig. 1), with some stems also covered with white patches. Disease severity was low, but the number of plants with symptoms was high.

As symptoms were clearly different from those caused by Leveillula taurica, symptomatic leaves were examined in a laboratory. Hyaline hyphae, conidiophores and conidia produced solitarily or apparently in chains were observed under the stereomicroscope (Fig. 2). Mycelium on the leaf surface was stripped off with adhesive tape and fungal structures were analysed dry using light microscopy. Conidia were ellipsoid-ovoid or doliform, 24-42 x 14-20 μm (Fig. 3), while conidiophores were 79-107 μm long with cylindrical foot-cells. On the basis of these characters, the pathogen was identified as Oidium neolycopersici (Kiss et al., 2001).

Pathogenicity tests were performed on five tomato plants (cv. Belle) in pots. Fragments of the diseased leaves were pressed onto previously wetted leaves of healthy plants. Inoculated plants were covered with polyethylene bags and incubated at 20/25 ⁰C with a 12-h photoperiod. Polyethylene bags and leaf fragments used for inoculation were removed after two days. First symptoms on leaves were observed five days after inoculation. Two weeks after inoculation, powdery mildew patches completely covered inoculated leaves, which became chlorotic. Interestingly, symptoms were also evident on stems of all inoculated plants. Morphological features of the powdery mildew fungus developed on inoculated plants were similar to those of Oidium neolycopersici. Infected leaves were deposited as herbarium specimens in the Department of Plant Pathology collection.

Oidium neolycopersici is present in many European countries, including Hungary, a country bordering Croatia (Mieslerová & Lebeda, 1999; Kiss et al., 2001). Tomato powdery mildew caused by O. neolycopersici can cause significant damage on greenhouse-grown tomatoes and is becoming increasingly important on field-grown tomato crops in many countries around the world (Jones et al., 2001). Most of the existing tomato cultivars have shown to be susceptible (Mieslerová & Lebeda, 1999). As tomato is among the most popular greenhouse crops in Croatian vegetable production, it is likely that this disease could become problematic and lead to economic losses in future. To our knowledge, this is the first report of O. neolycopersici on tomato in Croatia.