Eumusae leaf spot

Symptoms

Mycosphaerella eumusae has similar leaf spot characteristics to M. fijiensis and M. musicola. Primary lesions are brown streaks that expand to form large brown spots. This stage of the disease is the most recognisable and can be used to distinguish between the three Mycosphaerella leaf spot diseases. As the disease progresses, spots become grey in the centre but keep a brown border (Crous & Mourichon 2002).

The fungus:

Leaf spots amphigenous, initially visible as faint brown streaks, developing into oval or elliptical light brown lesions with grey centres and dark brown borders, coalescing to form large, brown necrotic areas under favourable conditions. Grey spots and patches are visible in necrotic areas, and lesions are surrounded by a chlorotic yellow zone. Pseudothecia amphigenous, predominantly hypophyllous, black, subepidermal, becoming slightly erumpent, globose, up to 80 µm diam., apical ostiole 10–15 µm wide; wall consisting of 2–3 layers of medium brown textura angularis. Asci aparaphysate, fasciculate, bitunicate, subsessile, obovoid, straight or slightly incurved, 8-spored, 30–50 × 9–15 µm. Ascospores tri- to multiseriate, overlapping, hyaline, guttulate, thick-walled, straight, obovoid with obtuse ends, widest in the middle of apical cell, medianly 1-spetate or basal cell slightly longer than apical cell, tapering towards both ends, but with more prominent taper towards lower end, (11–)12–13(–16.5) × (3–)3.5–4(–4.5) µm. Spermogonia predominatly hypophyllous, subepidermal, substomatal, globose, dark brown, up to 75 µm diam. Spermatia hyaline, rod-shaped, 3–6 × 1–2 µm. Mycelium internal, pale brown, consisting of septate, branched, smooth hyphae, 1.5–2.5 µm wide. Caespituli sporodochial, subepidermal, substomatal, predominantly epiphyllous, grey, up to 100 µm wide. Conidiophores aggregated in dense fascicles arising from the upper cells of a brown stroma up to 70 µm wide; conidiophores subcylindrical, smooth, hyaline or pale brown below, 0–3-septate, straight to geniculate-sinuous, unbranched or branched below, 10–25 × 3–5 µm. Conidiogenous cells terminal, unbranched, hyaline, smooth, tapering to flat-tipped apical loci, proliferating sympodially, or 1–4 times percurrently near the apex, 10–20 × 3–4 µm; scars inconspicuous. Conidia solitary, subhyaline to pale olivaceous, thick-walled, smooth, subcylindrical, apex obtuse, base subtruncate, straight to variously curved, 3–8-septate, (18–)30–50(–65) × (2–)2.5–3 µm; hila inconspicuous (Crous & Mourichon 2002).

Notes:

Two other virulent Mycosphaerella leaf spot diseases of banana are known. M. fijiensis Morelet (anamorph: Pseudocercospora fijiensis (Morelet) Deighton; black Sigatoka) and M. musicola R. Leach (anamorph: Pseudocercospora musae (Zimm.) Deighton; Sigatoka disease).

The teleomorphic stages of the Mycosphaerella leaf spot diseases on banana are morphologically similar and cannot yet be used as a method of diagnosis (Carlier et al. 2000). The anamorphic forms are distinctive and can be used for morphologic identification (Crous & Mourichon 2002):

Table: Anamorphic characteristics of Musa leaf spot fungi.