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Symptoms and signs

The first symptoms of black Sigatoka disease are tiny, chlorotic spots that appear on the bottom (abaxial) surface of the 3rd or 4th open leaf. The spots grow into thin brown streaks that are limited by leaf veins (Figure 3). The color of the streaks becomes darker, sometimes with a purple tinge, and visible on the top (adaxial) surface. The lesions then enlarge, becoming fusiform or elliptical, and darken to give the characteristic black streaking of the leaves (Figure 4). Adjacent tissue often has a water-soaked appearance, especially under conditions of high humidity.


Figure 3	Figure 4

When the disease severity is high, large areas of the leaf may become blackened and water-soaked. On the necrotic tissue, numerous, tiny, black, globose fruiting bodies (pseudothecia) containing sac-like structures (asci) filled with ascospores will emerge from the underside of the leaf (Figure 5).

Figure 5

Black Sigatoka, first reported in Fiji in 1964, gets its name from appearing as a different form of yellow Sigatoka. It has black color in the young streaks and spots and lacks the distinct yellow halo that is present in young streaks of yellow Sigatoka. Although the lesions of yellow Sigatoka (Mycosphaerella musicola) may look similar, one can distinguish between the yellow and black Sigatokas by looking for fungal hyphae producing asexual spores (conidiophores). Also, the two species may be distinguished microscopically from each other by examining conidiophore structure. Mycosphaerella fijiensis produces conidiophores in clumps (sporodochia) and has basal scars on the conidium and conidiophore at their points of attachment. Mycosphaerella fijiensis produces most of its asexual spores (conidia) and structures producing male sexual spores (spermagonia) on the underside of the leaf, whereas M. musicola produces conidia predominantly on the upperside of the leaf. Symptoms of yellow Sigatoka (Figures 6 and 7) typically develop more slowly than symptoms of black Sigatoka (Figure 8) (See below: Yellow and Black Sigatoka Compared). In addition, diagnosis can also be accomplished through the use of polymerase chain reaction (PCR).


Figure 6	Figure 7

Figure 8

If left unchecked, black Sigatoka will progress through the plant's leaf surface area, greatly reducing photosynthetic capability and thus yield (Figure 9).

Figure 9

Black and Yellow Sigatoka Compared

Pathogen
YELLOW SIGATOKA	BLACK SIGATOKA
Mycosphaerella musicola (Pseudocercospora musae)	Mycosphaerella fijiensis (Pseudocercospora fijiensis)
conidiophores formed in dense clusters (sporodochia) on dark stromata on both leaf surfaces conidiophores straight, usually nonseptate and unbranched, no spore scars conidia uniform thickness for full length, 1-5 septate, no distinct basal scar	conidiophores formed singly or in small groups (2-5) on lower leaf surface conidiophores straight or bent, 0-3 septate and occasionally branched, slightly thickened spore scars conidia taper from base to apex, 1-6 septate, distinct basal scar
Hosts
YELLOW SIGATOKA	BLACK SIGATOKA
Bananas (AAA) generally susceptible; most cooking bananas and plantains (AAB and ABB) moderately to highly resistant	Most dessert bananas, cooking bananas, and plantains susceptible
Symptoms
YELLOW SIGATOKA	BLACK SIGATOKA
early streak pale yellow streaks appear on leaf numbers 4-5 (unsprayed Cavendish)	early streak dark brown streaks appear on leaf numbers 2-4 (unsprayed Cavendish)
Epidemiology
YELLOW SIGATOKA	BLACK SIGATOKA
more common in cooler environments inoculum consists of both conidia (water-dispersed) and ascospores (wind-dispersed) conidia first appear in the mature spot stage produce more than 30,000 condia per spot conidia not dislodged by wind mature ascospores produced 4 weeks after the appearance of streaks	more common in warmer environments windborne ascospores are the major inoculum conidia first appear in early streak stage produce about 1200 condia per spot conidia both water- and wind-dispersed mature ascospores produced 2 weeks after the appearance of streaks

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