INJURY TO ONIONS BY THRIPS-TABACI (THYSANOPTERA, THRIPIDAE) AND ITS ROLE IN THE DEVELOPMENT OF PURPLE BLOTCH

Greenhouse studies were conducted to determine the role of Thrips taba ci Lindeman injury to onion, Allium cepa L., in the invasion of the fu ngal pathogen Alternaria porri (Ellis) and the subsequent development of the disease purple blotch. Individually caged onions were infested with 25 thrips per plant and inoculated 20 d later with A. porri (1,00 0 spores per ml of H2O). Onions were rated 6 and 7 d after inoculation . Plants infested with thrips and inoculated with A. porri developed m ore and larger lesions that coalesced as the plant matured. Older leav es were more susceptible to A. porri infection in the absence of thrip s injury. Purple blotch infection shifted to younger leaves as a resul t of thrips injury to onion, and more tissue was killed by thrips in t he presence of purple blotch. Plant growth was negatively correlated w ith thrips feeding injury. Scanning electron microscopy was used to de termine morphological changes in leaf tissue resulting from thrips fee ding injury and the development of purple blotch. In the absence of th rips injury, A. porri enters onion leaf tissue through stomates and di rectly through the epidermal cell layer. When both thrips injury and p urple blotch are present, leaf tissue is extremely necrotic and A. por ri appears to use areas of insect damage as alternative penetration si tes. Thus, the feeding wounds caused by thrips enhance entry and devel opment of the disease by providing alternative penetration sites for A . porri.