The Development of Infection Structures of Sderotium cepivorum on Onion

Abstract

Growth of Sderotium cepivorum mycelium on root tissue differed from that on stem tissue. Hyphae grew along the lines of the longitudinal epidermal cell walls often producing side branches which resulted in a distinctive pattern of growth. Penetration occurred mainly between anticlinal wall junctions with occasional direct penetration through the periclinal wall. Growth on the surface of the stem resulted in the formatíon of donne shaped infection cushions, arising from repeated dichotomous branching of hyphal tips. Penetration of the stem tissue occurred solely from these structures. The results of experiments using artificial membranes and surface replicas indicated that the stimulus for attempted penetration was chemical in nature but that the nature of the infection structure produced was determined by the relative strength of the tissue under attack.

Histology of Sclerotium cepivorum infection of onion roots and the spatial relationships of pectinases in the infection process

“…(1989a ) noted infection cushion formation on leaves and the stem base; in this study infection cushions were also noted on roots. Stewart et al . (1989b ) suggested that infection cushion formation on leaf and stem, but not root tissue, was in response to the greater resistance of these tissues to penetration because of the cuticle.…”

“…Sclerotium cepivorum sclerotia can survive in soil for more than 20 years ( Coley‐Smith et al ., 1990 ) and produce mycelium in response to the presence of Allium root exudates ( Coley‐Smith & Holt, 1966). The process of fungal penetration has been described ( Abd El‐Razik et al ., 1973 ; Stewart et al ., 1989a , b). Following penetration of the root epidermis, S. cepivorum invades cortical parenchyma both intra‐ and intercellularly, causing extensive tissue degradation ( Abd‐El‐Razik et al ., 1973 ).…”

“…Following penetration of the root epidermis, S. cepivorum invades cortical parenchyma both intra‐ and intercellularly, causing extensive tissue degradation ( Abd‐El‐Razik et al ., 1973 ). Destruction of epidermal cell walls beneath the stem base cuticle occurs two to three cells ahead of infection hyphae ( Stewart et al ., 1989b ). Sclerotium cepivorum produces the phytotoxin oxalic acid ( Stone & Armentrout, 1985) and a range of extracellular enzymes that degrade the cell wall, including polygalacturonases (PG, EC 3.2.1.15, Mankarios & Friend, 1980; Favaron et al ., 1993 ), pectinesterases (PE, EC 3.2.1.11, Abd El‐Razik et al ., 1974 ), cellulases, arabanases and xylanases ( Mankarios & Friend, 1980).…”

Signaling for Infection Structure Formation in Fungi

“…Plant extracts are the most frequently cited chemical factor inducing infection cushion formation in R. solani (Flentje et al, 1963;Marshall and Rush, 1980;Stockweiland Hanchey, 1983) as weil as in Sclerotium cepiyorum (Steward et al, 1989). Likewise, leaf diffusates were deinonstrated to not only stimulate lateral germination of Botrytis conidia, but also to enhance appressorium formation (C1ark and Lorbeer, 1976a,b).…”

Rind distortion of lemon caused byBotrytis cinereaPers

“…De Bary on bean petals (Abawi et al 1975), and Sclerotium cepivorum Berk, on the stems of onion seedlings (Stewart et al 1989). It has been suggested that the type and complexity of the infection structure developed by this group of organisms is related to the degree of development of the original epiphytic hyphae (Garcia-Arenal & Sagasta 1980), the nutrient level available to the surface hyphae (Akutsu et al 1980), or the hardness of the relative strength of the tissue under attack (Stewart et al 1989). In the instance of infection on lemon, infection cushions were only formed by hyphae developing from a substantial food base such as floral remnants or an agar block.…”