First report of Oidium neolycopersici on tomatoes in Turkey

Tomato plants showing typical signs of powdery mildew were received in April 2006 from a hydroponic greenhouse situated in western Turkey. Dense, irregular white patches observed on the upper surfaces of the leaves and on the stems of the plants (Fig. 1) could be easily differentiated from the symptoms caused by Leveillula taurica. L. taurica is known as the unique agent of tomato powdery mildew in Turkey up to now and causes white powdery masses appearing just under the chlorotic spots that are produced on the adaxial surface of the leaves.

To determine the morphological characteristics of the pathogen, surface mycelium was removed with small strips of clear adhesive tape and examined using light microscopy. Microscopic observations revealed ellipsoid-ovoid or doliform conidia that measured 31-57 x 15-27 µm (mean: 33.7 x 17.4 µm; n=50) germinating with one short germ tube terminating in simple apices. Conidiophores were straight, with cylindrical foot-cells measuring 47-78 µm (mean: 53.8 µm), followed by two or three short cells. The total length of the conidiophores measured 73-104 µm (mean: 87.4 µm). Conidia were solitary or sometimes in short pseudo-chains of 2-4 conidia (Fig. 2). Based on these characteristics the fungus was identified as Oidium neolycopersici (Kiss et al., 2001).

To confirm the pathogenicity of the fungus, disease free tomatoes plants (20 plants of each cultivars SC 2121 and H 2274) were inoculated at the five to six true-leaf stage with conidia falling from diseased tomato leaves hanging above them. Plants were kept in a polyethylene chamber placed in a greenhouse cabinet at 21 ± 1⁰C and a 14-h photoperiod for five days. The polyethylene chamber was then removed and the plants were grown in the greenhouse. The first white fungal colonies appeared on the leaves of the inoculated plants 7 days after inoculation and after 14 days, a powdery mildew, exhibiting the same morphological features, was observed on all of the tomatoes plants.

This is believed to be the first report of powdery mildew caused by Oidium neolycopersici on tomatoes in Turkey. This disease has the potential to cause economic losses (Jones et al., 2001) and may become a problem in greenhouse-grown tomatoes in the near future.