The catcher in the gene

Evidence finally links phytoplasmas to root wilt disease in coconut trees

 
By Manupriya
Last Updated: Saturday 04 July 2015

imageIt was May 2009. S Abdul Rahman at Cumbam village in Tamil Nadu’s Theni district was worried about his coconut plantations. The palm trees were drying at the roots, leaflets were bending and the older leaves were completely dry. These were dreaded symptoms he knew well.

Neighbouring farms were also facing the same predicament. Yield losses were increasing and farmers with small land holdings were worse off.

Come summers, farmers like Rahman dread the winds that bring this perplexing disease, first observed in 1874 in Kerala. Known as root wilt, it has spread across Kerala, to Tamil Nadu where it is called the Kerala wilt disease. Farmers go all out to make sure there is high irrigation, with good drainage, removal of infected plants and mixed cropping to reduce the effects of this debilitating disease which has no cure. Yet, statistics show it has become a major production constraint causing a yield loss of 968 million coconuts per year in south India.

As tree after tree got infected, Rahman knew saving them was not in his hands anymore; he sought help. He talked to certain acquaintances at the Central Plantation Crop Research Institute (cpcri) in Kasaragod in Kerala. His timing was good because R Manimakelai, a coconut researcher from cpcri, was collecting samples from diseased plants in Tamil Nadu at the time. Several plantations were located in Cumbam; Rahman’s also figured in her study.

The first thing Manimakelai did was confirm the root wilt disease. But what causes the disease is not fully understood nor is the agent characterized even after decades of study. R Manimakelai’s research on what caused the infection led to a phytoplasma.

Bacteria-like trippers
Phytoplasmas are bacteria-like organisms but unlike bacteria they hail from a group that does not have a cell wall and, therefore, no fixed shape. The organisms are known to live and accumulate in the saliva of insects. Transmitted via bites, infection is transferred in leaps and bounds. Their most unique feature is they cannot be grown in artificial conditions and one has to depend on infected plants to isolate them. “Even their titre (level) can vary according to the season and plant species,” said plant virologist V K Baranwal at the Indian Agricultural Research Institute in Delhi and a co-author of Manimakelai’s paper published in Plant Diseases this May.

image
Effort to detect
the coconut root
wilt phytoplasma
using the gene
amplification
method did not
yield any results
so far
—R Manimakelai
To better understand the phytoplasma behind the root wilt of coconut, Manimakelai and her group first isolated dna from leaf tissues of both infected and uninfected plants growing in different regions of India. Low titres of phytoplasma in the plants makes the otherwise simple process quite difficult. Once they isolated the dna, they tried to amplify a specific gene using the polymerase chain reaction (pcr). The gene would indicate the presence of the phytoplasma and this time they found it in the infected plants. Manimakelai was jubilant. “Efforts to detect phytoplasma in root wilt of coconut using the gene amplification method in the past one decade have been unsuccessful,” said Manimakelai. She now wants to use this method to make a robust kit for early detection. This means the moment a leaf wilts Rahman will know what to prepare himself for.

While the work seems promising, care should be taken while interpreting the results, said Matthew Dickinson, an expert on phytoplasma at the University of Nottingham in the UK. “In other countries, a combination of phytoplasma and fungi was associated with coconut root wilt diseases, so control may be a lot more complex than this report suggests,” he said.

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