Filamentous and Mycological Don’t let Venturia inaequalis flowering crabapples (Malus spp.), hawthorn (Crataegus spp.), mountain put its footprint on ash (Sorbus spp.), firethorn (Pyracantha your apples spp.), and loquat (Eriobotrya japonica). Pear (Pyrus spp.) is infected by a related Alan R. Biggs, fungus, Venturia pirina, which causes West Virginia University nearly identical symptoms. The apple scab pathogen was first hile the rest of us are named Spilocaea pomi in 1819 by the hunting morels in Swedish botanist, Elias Fries; but it was abandoned apple orchards, G. Winter, in 1880, who first placed Wapple growers and home orchardists specimens of the sexual and asexual are waging battle against several fungal stages from apple leaves and labeled pathogens – the most important one the fungus as a species of Venturia, being Venturia inaequalis, the apple although he did not demonstrate the scab pathogen. The fungus also affects relationship between the two stages. Aderhold, in 1897, was the first to do this, so the correct nomenclature for the NOW in paperback! fungus is Venturia inaequalis (Cooke) Winter amend. Aderhold (MacHardy, 1996). Barr (1968) provides the most thorough discussion of the Venturiaceae in North America. Signs of the apple scab fungus are most noticeable on leaves and fruit although other plant parts can become infected. The first infections, often called lesions, are seen in the spring and are usually on the underside of the first-formed leaves because they develop with their undersides facing Figure 1. out, prior to unfolding. Once the leaves leaves are usually smaller because leaves unfold, the upper surfaces also become become more resistant to infection as vulnerable to infection. An infection they age. Affected tissues eventually may first appears as an area that is a lighter become distorted and puckered, and shade of green than the surrounding the leaf lesions may become ripped or leaf. The lesion is usually circular and torn. Lesions on the fruit are generally as it increases in size it becomes olive- more “scabby” in appearance, with a colored and velvety due to production of distinct margin (Fig. 3). The earliest asexual spores (conidia) (Figs. 1 and 2). noticeable symptoms on fruit are mycophilia.com Lesions that form on young leaves are water-soaked areas which develop into variable in size, some more than 1 cm in velvety, green to olive-brown lesions. diameter. Infections that occur on older 34 FUNGI Volume 6:2 Summer 2013 Infections of young fruit usually cause damage to the apple foliage. Even today, (see Fig. 5, ascospores are stained red). fruit distortions leading to unusual in spite of the highly effective chemicals The unequal size of the cells of the scabby shapes often accompanied by and the resistant apple varieties that ascospores inspired the Latin species cracking (Fig. 4). Severely infected are available, apple scab causes greater epithet for the fungus, “inaequalis.” leaves or fruit will often drop from the economic losses of apples in North and They are 5 to 7 µm wide and 11 to 15 tree prematurely. When significant South America, Europe, and Asia than µm long. In the spring, when the over- defoliation for two or three years in a any other disease. wintered leaves become wet, the mature row occurs, the result can be weakened Venturia inaequalis is an pseudothecia swell and protrude from trees that are more susceptible to ascomycete fungus within the class the surface of the leaf. The ascospores freeze damage, insect injury, and other Loculoascomycetes, order Pleosporales, are forcibly ejected and carried by rain diseases (Biggs, 1990). family Venturiaceae. Loculoascomycetes and wind to the young blossoms and Venturia inaequalis has long been don’t form a true perithecium, but leaves of apple trees, where, under a problem on apples; symptoms of instead form their asci inside locules the appropriate set of conditions, the disease can be observed on fruit that form by dissolving or crushing they germinate and initiate infection. in paintings from the 15th and 16th the internal tissues of the stroma, There is only one cycle of ascospore centuries, with perhaps the earliest thus forming a pseudothecium (Fig. 5, production per year. record (ca. 1600) seen in a painting by courtesy American Phytopathological Like most other ascomycetes, V. Michel Angelo Caravaggio Christ and Society). Also, the walls of the asci are inaequalis reproduces asexually by the two disciples at Emmaus (National bitunicate in this class. The mycelium of producing spores called conidia. The Gallery, London) (MacHardy, 1996). The V. inaequalis is septate, and the nuclei conidial stage of the V. inaequalis has its frequent depictions of infected apples are haploid. own name, Spilocaea pomi. The conidia suggest that the disease was common The brief diploid stage in the life are single-celled, uninucleate, brown or and that the affected fruit was acceptable cycle of V. inaequalis occurs within olive in mass and are narrower at one in earlier times. All the commonly the pseudothecium in single hyphal end. They are 6 to 12 µm wide and 12 grown apple varieties were susceptible to cells, termed croziers, which give rise to 22 µm long. Conidia are produced the disease, and there were no chemical to the haploid ascospores following by specialized short hyphae called treatments to prevent the disease until meiosis. The asci are elongated, sac-like conidiophores. The conidiophores are the late 1800s. At that time, copper- structures, each of which contains eight formed on a dense mat of mycelium and sulfur-based fungicides provided ascospores in a linear arrangement. The that pushes up through the leaf cuticle protection if applied prior to infection, ascospores are brown, two-celled, and and ruptures it. It is the mass of conidia but the chemicals caused substantial have a characteristic “footprint” shape and conidiophores that causes the Nobody wants DEET in their chanterelles. Our all natural citronella soap and insect repellent keep you bug free. www.herbariasoap.com Figure 2. FUNGI Volume 6:2 Summer 2013 35 Figure 3. Figure 4. velvety appearance of the young lesions. with a characteristic “footprint” shape, on over-wintering leaves. Conidia are produced within the lesions 5 to 7 µm wide and 11 to 15 µm long; Habitat: worldwide wherever apples from nine to thirty days after initial conidia single-celled, uninucleate, brown are grown, particularly in cooler climate infection of the leaf, depending on the or olive in mass, narrower at one end, 6 to regions. temperature. Conidia are disseminated by 12 µm wide and 12 to 22 µm long. Fruiting season: spring, summer, fall. wind and by wind-blown and splashing Diagnostic macroscopic feature: olive- Prevention: sanitation is the best method of prevention, although it is difficult to implement on a large scale; removing all fallen leaves and fruit prior to the new growing season will disrupt the disease cycle; there are good cultivars with complete genetic resistance and these are recommended for home gardeners and organic growers; sanitation and timely applications of fungicides based on weather forecasts, plant development, and pathogen presence are the basis for commercial apple production. References Cited Barr, M.E. 1968. The Venturiaceae of North America. Canadian Journal of Botany 46: 799–864. Biggs, A.R. 1990. Compendium of Apple and Pear Diseases. Jones, A. L. and Figure 5. H.S. Aldwinkle, eds. APS Press, St. Paul; pp. 6–9. rain. Once a conidium lands on an apple green lesion with feathery margin on leaf, blossom or fruit, it adheres to the apple leaves; scabby lesions on fruits. Fries, E. 1819. Spilocaea pomi, Fries. surface and germinates in the presence Diagnostic microscopic feature: Novitiae Florae Svecicae 5: 61–80. of free moisture. The germination hypha ascospores have distinctive “footprint” MacHardy, W.E. 1996. Apple Scab penetrates the cuticle and establishes a shape. Biology, Epidemiology, and new infection. There can be many cycles Nutritional mode: obligate parasite Management. APS Press St. Paul; of conidial production and infection during pathogenesis and saprotroph 545 pp. within a single growing season, often during sexual reproduction. MacHardy, W.E. 1996. Apple leading to severe disease outbreaks Substrate: lesions with conidia on Scab Biology, Epidemiology, and termed epiphytotics. leaves, petioles, blossoms, sepals, fruits, Management. APS Press St. Paul; Spores: ascospores brown, two-celled, and pedicels; ascospores in pseudothecia 545 pp. 36 FUNGI Volume 6:2 Summer 2013.