Taro Leaf Blight

By Glenn I. Teves, UH County Extension Agent

Autumn started late, but came in with a roar. Blustery winds and horizontal rains have arrived and the forecast for this winter is rainier than normal conditions. What does this mean for gardeners and farmers? It means more weeds, more diseases, and the need for wind protection for most crops. It also means that with the cold weather comes a whole new set of crops to grow. Most of what you plant today won’t come out until mid January, but between now and then, there will be a lot of challenges to contend with.

For taro growers, this season signals the return of an old nemesis, taro leaf blight. This disease will melt away leaves, adversely affecting both the quality and quantity of taro. Poi made from diseased plants will lack the viscosity, or stickiness, found in high quality poi. Under ideal conditions, this disease is rampant and unstoppable, and we’ve seen our share of this disease on Molokai. It prefers cool weather, wet conditions, and no wind over extended periods of time. The wind is one thing that holds off the disease in Ho`olehua, but when Kona or west winds arrive, with the combination of cool weather it’s the perfect storm.

Taro leaf blight is believed to have caused famines in ancient times, and we still haven’t found a good way to control it. The disease has caused major epidemics in Dominican Republic, Puerto Rico, American Samoa, and most recently, West Africa. Taro is an important subsistence crop in many parts of the world, and vital to the survival of native people.

The University of Hawaii College of Tropical Agriculture and Human Resources (CTAHR) has attempted to cross Hawaiian taro varieties with leaf blight tolerant varieties from Pacific islands including Yap, Palau, Guam and Pohnpei. The challenge in breeding is to capture the good characters without the bad ones. Some of bad ones include runners on the corm, poor taste, and poor poi souring qualities. Taro with runners can take over a lo`i or water patch in a short time. To date, none of the hybrids possess enough resistance to keep the disease in check when conditions are ideal for the disease. The Hawaiian selections, originally numbering in the hundreds, are believed to originate from four to five original introductions to Hawaii and are therefore closely related. None of them show any tolerance to leaf blight. In fact, the softer leaves favored for making dishes such as squid luau are especially susceptible.

There are cultural measures to slow the disease. One way is to plant in windy areas when the disease is present.  Another way is to plant huli farther apart, such as more than two feet. Air circulation can curb the spread. The calcium status of the soil is also critical. An important component of the cell wall, low calcium will decrease the thickness of the cell wall making them more susceptible to leaf penetration by leaf blight. High doses of nitrogen can also create calcium deficiency. Lush growth makes for a fast-growing, floppy leaf that is very susceptible to leaf blight. Frequent, low doses of conventional nitrogen or organic sources with slow release nitrogen will help. For more information, you can download a publication on Taro Leaf Blight we completed this year at: ctahr.hawaii.edu/oc/freepubs/pdf/PD-71.pdf.