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Pathogen Biology

Venturia inaequalis is an ascomycete fungus because it produces sexual spores (ascospores) in a sac-like structure called an ascus (plural asci). The mycelium of V. inaequalis is septate, and the nuclei are haploid.

Sexual Reproduction
Venturia inaequalis occurs as two mating types. Both must be present in order for sexual reproduction to be initiated. Mating takes place in the early spring within infected fallen leaves left from the previous season. It consists of fusion of a male organ (antheridium) formed from a hyphal tip of one mate to a female receptive hypha (trichogyne) from the opposite mate. The trichogyne is attached to a coil of hyphae called the pseudothecial initial. During fertilization, a nucleus passes from the antheridium through the trichogyne into a cell at the base of the pseudothecial initial. After fertilization, the pseudothecial initial develops into a pseudothecium (Figure 7). The pseudothecium is a cavity located within a dense mat of fungal mycelium called a stroma. Inside this cavity the asci and ascospores are formed (Figure 8). The very brief diploid stage in the life cycle of V. inaequalis occurs within the pseudothecium in single hyphal cells (croziers) which give rise to the haploid ascospores following meiosis. The asci are elongated, sac-like structures, each of which contains eight ascospores in a linear arrangement (Figure 9). The ascospores are brown, two-celled, and have a characteristic "footprint" shape (Figure 10). The shape of the ascospores inspired the Latin name for apple scab, "inaequalis", which refers to the unequal size of the cells. They measure between 5 and 7 µm wide and between 11 and 15 µm long. In the spring, when the fallen leaves become wet, the mature pseudothecia swell and protrude from the surface of the leaf (Figure 11). The ascospores are released and carried by rain and wind. If they land on the young blossoms and leaves of apple trees, they germinate and initiate infection. There is only one cycle of ascospore production and infection within a season.

Figure 7	Figure 8
Figure 9	Figure 10
Figure 11

Asexual Reproduction
Like most other ascomycetes, V. inaequalis reproduces asexually by producing spores called conidia. The conidial stage of the V. inaequalis lifecycle has its own name, Spilocaea pomi. The conidia are single-celled, uninucleate, brown or olive in mass but lighter when viewed individually under a microscope, and are narrower at one end than the other (Figure 12). They measure between 6 and 12 µm wide and between 12 and 22 µm long. Conidia are produced by specialized short hyphae called conidiophores. The conidiophores are formed on a dense mat of mycelium which pushes up through the leaf cuticle and ruptures it (Figure 13). It is the mass of conidia and conidiophores which causes the velvety appearance of the young lesions. Conidia are produced within the lesions from nine to thirty days after initial infection of the leaf, depending on the temperature. Conidia are disseminated by wind and by wind-blown and splashing rain. Once a conidium lands on an apple leaf, blossom or fruit, it adheres to the surface and germinates, provided the tissue is wet. The germination hypha penetrates the cuticle and establishes a new infection. There can be many cycles of conidial production and infection within a single growing season. When infection of fruit occurs late in the fall it may not be detected until the fruit has been stored for several months.

Figure 12

Figure 13

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