Phoma spp.*

(Taro Shot Hole Disease)

General

• Mushroom widely distributed in the South Pacific Islands, often mistakenly named "Phyllosticta" in the field. * Two species of Phoma seem to appear: an unidentified one, another named Phoma colocasiae .
• Mainly induce symptoms on taro foliage, and serious damage only during humid conditions associated with cool temperatures.
• Disease observed in the field.

• Susceptible botanical family(s)

• Production areas affected

• Organs attacked

Symptoms

• Symptoms : 
  • Brown leaf spots of limited size at first, extending and gradually becoming necrotic on the middle leaves of plants (Figure 1). Rounded in shape, these lesions are surrounded by a bright yellow halo (figures 2 and 3) and measure 2 to 4 cm in diameter. They also have fairly spaced concentric circle patterns.
  • Disappearance of altered tissues at the level of the spots giving way to more or less large and scattered holes, and giving the blade a riddled appearance.
  • Reduction of the photosynthetic potential of the plants causing the reduction in the size of the corms produced in the case of early and severe attacks.

• Signs : numerous and tiny dark brown to black globular masses (figure 4), pycnidia visible especially on the upper side of the lamina. These structures contain small, uni- or bi-cellular, hyaline capsule-shaped spores, usually observed microscopically in bundles following their emergence from the pycnidia (Figure 7).
• Possible confusion : taro downy mildew ( Phytophthora colocassiae ) which is a disease absent from New Caledonia, corynesporiosis , etc.
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Biology

• Conservation : in the form of mycelium on plant residues.
• Infection : after germination of ascospores or conidia, the fungus invades the tissues between the cells of the parenchyma.
• Sporulation : the mycelium present in the infected tissues can produce pycnidia (figure 5) containing pycnidiospores (figures 6 and 7), but also pseudothecia containing ascospores (teleomorph - sexual form of the fungus).
• Dissemination : ascospores carried by the wind over long distances (8 to 10 km), also by conidia from the pycnidia following splashes of water (rain, dew, etc.).
• Favorable conditions : few requirements.

Protection

• Lengthen the rotations in order to reduce the stock of inoculum.
• Choose resistant varieties .
• Use healthy plants and control their quality.
• Do not plant during the most favorable season for the fungus.
• Ensure good drainage of cultivated plots.
• Encourage aeration and sunshine of the crop (planting density and choice of plot according to its orientation).
• Do not allow workers to work while vegetation is wet.
• Eliminate plant residues , during cultivation following the various cultivation operations, and at the end of cultivation after uprooting the plants. They will have to be destroyed.