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Disease Cycle and Epidemiology

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Sporangia are detached from sporangiophores at the edge of lesions and spread by rain splash and wind-blown rain. They germinate on leaves and petioles, or are washed into the soil where they can infect taro corms.

If the A1 and A2 mating types are present, antheridia and oogonia can form, producing oospores. Oospores may act as survival structures in plant tissue between crops.

The pathogen survives as mycelium in plant tissues or as encysted zoospores in soil. Infected planting material, along with sporangia and zoospores in wind-driven rain, are the main sources of inoculum in new plantings.

Epidemiology

Sporangia and zoospores are spread by rain splash and wind-blown rain between plants (alloinfection) (Figure 16), or within the same plant (autoinfection) (Figure 17). Due to the slanted aspect of taro leaves and their thick, waxy cuticle, many sporangia and zoospores are washed into the soil or splashed onto petioles (Figure 18). Petiole lesions grow rapidly and may produce reddish-orange exudates (Figure 19). They should not be confused, however, with the small lesions caused by the taro planthopper (Tarophagus proserpina), which also produce exudates (Figure 20). In wetland (flooded) taro production, sporangia and zoospores are spread between plants and fields by paddy water.

Figure 16	Figure 17
Figure 18	Figure 19
Figure 20

The warm, wet climate of the tropics allows taro to be grown throughout the year, ensuring a continuous supply of host plants. Taro is vegetatively propagated, and P. colocasiae is spread from field to field and over long distances by infected planting material. Corms left in the field after harvest can also serve as inoculum sources in newly planted taro plots. Mycelium usually lasts less then five days in the soil, but encysted zoospores can survive for up to 3 months. Oospores and chlamydospores are formed in culture and may act as survival structures in infected plant tissue or soil, but have not been reported in nature.