Squash silver leaf: biochemical aspects of this disorder and its influence on the development of Sphaerotheca fuliginea on the tissues

Author(s)

Fabiana Vieira Hoto

• Document type: Master's Dissertation
• Press: Piracicaba.
• Institution: Universidade de São Paulo (USP). Escola Superior de Agricultura Luiz de Queiroz (ESALA/BC)
• Defense date: 1999-01-29
• Advisor: Sergio Florentino Pascholati

Abstract

During the last years, Cucurbita species have been largely infested by Bemisia tabaci (B-byotipe), commonly known as whitefly. This insect induces the disorder known as silver leaf which affects the upper surface of leaves and alters metabolic processes such as photosynthesis and respiration. On the other hand, recently, on squash plants kept under field or greenhouse conditions, it has been observed a reduction in the incidence of powdery mildew, caused by the ectoparasite S. fuliginea, on silvered leaves when compared to normal green leaves. In this work, some metabolic aspects of plants exhibiting silver leaf were examined as well as the influence of this disorder on growth of S. fuliginea on the leaf tissue. Regarding the metabolism, the results showed a reduction in chlorophyll content in all squash plants (Cucurbita pepo, Cucurbita moschata and Tetsukabuto hybrid - C. moschata x Cucurbita maxima) with silvered leaves suggesting an influence of this disorder on photosynthesis. In the case of C. pepo, the content of protein, carbohydrate and phenols was reduced in leaves exhibiting silver leaf. In contrast, with the exception of protein content in C. moschata, that was the same as in the normal green leaves of plants from the same species, the content of carbohydrates and phenols was higher in leaves of C. moschata and Tetsukabuto hybrid (the protein content was higher in the hybrid too) with silver leaf symptoms. Although these results seem to be conflicting, they indicate that the silver leaf disorder in squashes, regardless of the typical symptoms exhibited by the different plant materials, can cause metabolic changes that do not follow a standard pattern. Microscopical observations of C. pepo leaf cross sections exhibiting the disorder also pointed out changes in the structure of the mesophyll that were seen as new intercellular spaces and disorganization of parenchyma cells. Regarding the conidia of S. fuliginea on leaves of C. pepo with silvering symptoms, the results showed a high reduction in germination (74%) and appressorium formation (84%) when compared to normal green leaves. In contrast, for the hemibiotroph fungus Colletotrichum gloeosporioides f. sp. cucurbitae, causal agent of anthracnose, conidium germination and appressorium formation were not significantly altered in silvered tissue. These observations point out for the possible existence of mechanisms in silvered leaves acting selectively, during the pre-penetration stage, against S. fuliginea,a biotroph organism. Thus, concerning the development of the powdery mildew on squash plants exhibiting silvered leaves, it can be suggested that the lower tissue colonization by the fungus, seen as the absence or reduction of the signals of the plant pathogen on leaf surface, is due to the reduction in conidium germination and appressorium formation as well as to structural changes in leaf mesophyll. (AU)