Phytophthora colocasiae (taro leaf blight)

The spread of P. colocasiae between countries can be controlled through quarantine and biosecurity (SPS) measures (see Detection and Inspection). Quarantine measures were used to control the spread of P. colocasiae between the two islands of Samoa (Hunter et al., 1998).

In Samoa, a public awareness campaign (radio, television, videos and print media) was conducted to coincide with the spraying and quarantine measures. Information on disease symptoms, epidemiology including disease spread, disease control and effects on taro production were provided. However, there was no assessment as to the impact of this awareness campaign (Hunter et al.,1998). A study carried out in Nsukka, Nigeria recommended that extension agents should focus on an awareness amongst farmers of TLB disease (Ayogu et al., 2015).

Resistance has been identified in germplasm collections of several countries where TLB has been present for a long time, including the Philippines, Vietnam, Thailand, Malaysia, Indonesia and India (Ivancic and Lebot, 2000). Screening of germplasm from Palau revealed over 20 taro varieties with resistance to TLB (Singh et al., 2012). Several countries have embarked on breeding programmes which basically involve crossing resistant varieties with superior (high yielding and tasting) local taro varieties. In Samoa, farmers selected clones derived from crosses between initially local (Samoan) cultivars and those from Palau and the Federated States of Micronesia. Further improvement of resistance was achieved by introducing Asian varieties into the programme (Iosefa et al., 2012). This programme enabled the restoration of the Samoan taro export market which had been destroyed by taro leaf blight in the early 1990s.

Four varieties of taro, with the desirable qualities of some traditional Hawaiian landraces, were identified as high yielding and disease-resistant in Hawaii after multi-year, multi-location field trials. DNA fingerprints of the new genotypes distinguish them from the traditional landraces (Paudel et al., 2023). In Vanuatu, genotypes from the two major gene pools (Asia and Pacific) were combined to generate a wide genetic base (Lebot et al., 2003). Fourteen countries from America, Africa, Asia and the Pacific evaluated a selection of 50 indexed genotypes in vitro representing significant genetic diversity. On-station trials led to the distribution of best genotypes to farmers. Introduced genotypes were successfully crossed (controlled crossing) with local cultivars and new hybrids were produced. Seeds of these hybrids were shared internationally providing significant allelic diversity in different countries (Lebot et al., 2018).

Fiji is still free of taro leaf blight but remains vulnerable to the disease due to its location, climatic conditions, increasing trade, sea and air travel to other Pacific Island countries. Taro is one of the major export commodities and is vital for food security and livelihoods. Taro varieties, resistant to the disease, have been developed in an effort to prepare farmers should the disease appear in Fiji.

Cultural practices, such as removal of infected leaves during the early stages of disease development, tend to be more effective in small plantings, such as subsistence taro gardens (Putter, 1976). Other cultural practices include wide spacing of plants to reduce spread of disease, using sites surrounded by forest as a barrier to disease spread, isolation of new crops from those that are diseased, and the use of planting material free from disease. Intercropping of taro with other crops may help in reducing disease. Taro intercropped with sorghum and pearl millet as a single row or double rows was shown to be more effective than intercropping with okra and maize in managing the disease (Sugha and Gurung, 2006). Disease severity was found to be consistently higher in taro mono-cropping than in a taro/maize intercropping system (Amosa and Wati, 1997). Knowledge of P. colocasiae and the conditions that favour its proliferation can assist in managing the disease, for example, some professional taro growers in Thailand avoid serious TLB infections by planting during the dry season (Singh et al., 2012).

Biological control agents can have some impact on the disease. The fungus Trichoderma has shown promise as a biocontrol agent during in vitro and in vivo studies, with the most potent strain being Trichoderma harzianum (Moïse et al., 2018). Rhizobacteria strains have also been shown to be effective, significantly suppressing the growth of P. colocasiae in greenhouse trials (Kelbessa et al., 2022). Essential oils, for example, cinnamon oil, inhibited mycelial growth, zoospore germination and sporulation of the fungus and also leaf necrosis under laboratory conditions (Hong et al., 2021). Citrus aurantiifolia essential oil has also been shown to have an inhibitory effect on mycelial growth, sporangium production and leaf necrosis under laboratory conditions (Tchameni et al., 2018).

Due to the variable regulations around (de-)registration of pesticides, we are for the moment not including any specific chemical control recommendations. For further information, we recommend you visit the following resources: