Infection process ofEntomosporium mespilion leaves of saskatoon,Amelanchier alnifolia

Abstract

The infection process for Entomosporium leaf spot of saskatoon (Amelanchier alnifolia) was evaluated with scanning electron microscopy. Following treatment with a conidiospore suspension of Entomosporium mespili, detached leaves of saskatoon cultivars were fixed at various times during disease development (2, 5, 8, and 12 days postinoculation). Scanning electron microscopy showed that conidiospore germination occurred rapidly, usually by the extrusion of a single major germ tube from the larger of two body cells. Fungal penetration of the host cuticle involved appressoria, produced directly below the germinating spore or at germ tube termini. Subspore appressoria were most common and resulted in the earliest formation of leaf acervuli. Conidiospore germination and penetration occurred more rapidly on leaves of the susceptible cv. Buffalo, than on leaves of resistant cv. Success. At 2 days postinoculation, 43% of germinating conidiospores had penetrated the leaf surface of cv. Buffalo, compared with 12% on leaves of cv. Success. Acervuli bearing large quantities of conidiospores, were produced as the result of conidiogenesis from a subcuticular stroma of hyphae. Acervuli formed on leaves of all saskatoon cultivars, but cultivars differed in acervulus surface area and number of acervuli per leaf.Résumé : Le processus d'infection dans le cas de la tache des feuilles (causée par un Entomosporium) de l'amélanchier à feuilles d'aulne (Amelanchier alnifolia) fut étudié à l'aide du microscope électronique à balayage. À la suite d'un traitement avec une suspension de conidiospores d'Entomosporium mespili, des feuilles détachées de cultivars d'amélanchier furent fixées à différentes périodes du développement de la maladie (2, 5, 8 et 12 jours après l'inoculation). Le microscope électronique à balayage montra que la germination des conidiospores a lieu rapidement, habituellement par l'extrusion d'un tube germinatif unique à partir de la plus grosse des deux cellules somatiques. La pénétration fongique de la cuticule de l'hôte implique des appressoria qui sont produits directement sous la cellule en germination ou à l'extrémité du tube germinatif. Les appressoria sous-sporaux étaient plus communs et étaient responsables de la formation des premiers acervules sur les feuilles. La germination et la pénétration des conidiospores a eu lieu plus rapidement sur les feuilles du cultivar sensible Buffalo que sur les feuilles du cultivar résistant Success. Au jour 2 après l'inoculation, 43% des conidiospores en germination avaient pénétré la surface des feuilles de cv. Buffalo, par rapport à 12% sur les feuilles de cv. Success. Les acervules portant de grandes quantités de conidiospores furent produits à la suite de la conidiogénèse issue d'un stroma subcuticulaire d'hyphes. Les acervules se sont formés sur les feuilles de tous les cultivars d'amélanchier, mais des différences entre les cultivars furent observées quant à l'étendue de la surface et au nombre d'acervules par feuille.

Estimating inoculum production relative to percent leaf area infected and environmental conditions required for stimulating release ofEntomosporium mespiliconidia onAmelanchier alnifolia

“…A dry period may help to desiccate the surface of mature acervuli. Rainwater would then be able to wash conidia from the surface of the exposed acervuli (Ronald and St-Pierre 2002).…”

“…A slender appendage is commonly found on the apical and lateral cells. The production of conidiospores was shown to cause rupture of the acervuli surface and permit release of the conidia contained within (Ronald and St-Pierre 2002). Wind and rain splash are the critical components in epidemic development of many diseases (Madden 1997), and although no specific studies were conducted, Lange and Bains (1995) have observed that heavy rainfall is the most important environmental factor contributing to entomosporium leaf and berry spot in saskatoon orchards.…”

“…Wind and rain splash are the critical components in epidemic development of many diseases (Madden 1997), and although no specific studies were conducted, Lange and Bains (1995) have observed that heavy rainfall is the most important environmental factor contributing to entomosporium leaf and berry spot in saskatoon orchards. Once released, conidia of E. mespili can germinate within 6 to 18 h and then form appressoria, which penetrate the cuticle of leaves (Baudoin 1986;Ronald and St-Pierre 2002;van der Zwet and Stroo 1985). Disease symptoms first appear on leaves as small brown spots that develop yellow halos.…”