Injury to Onions by Thrips tabaci (Thysanoptera: Thripidae) and Its Role in the Development of Purple Blotch

Abstract

Greenhouse studies were conducted to determine the role of Thrips tabaci Lindeman injury to onion, Allium cepa L., in the invasion of the fungal 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 (1,000 spores per ml of H2O). Onions were rated 6 and 7 d after inoculation. Plants infested with thrips and inoculated with A. porri developed more and larger lesions that coalesced as the plant matured. Older leaves were more susceptible to A. porri infection in the absence of thrips injury. Purple blotch infection shifted to younger leaves as a result of thrips injury to onion, and more tissue was killed by thrips in the presence of purple blotch. Plant growth was negatively correlated with thrips feeding injury. Scanning electron microscopy was used to determine morphological changes in leaf tissue resulting from thrips feeding injury and the development of purple blotch. In the absence of thrips injury, A. porri enters onion leaf tissue through stomates and directly through the epidermal cell layer. When both thrips injury and purple blotch are present, leaf tissue is extremely necrotic and A. porri appears to use areas of insect damage as alternative penetration sites. Thus, the feeding wounds caused by thrips enhance entry and development of the disease by providing alternative penetration sites for A. porri .