Bacterial leaf stripe disease of arecanut (arecacatechu) caused by xanthomonas areoae

Abstract

The bacterial leaf stripe disease of Areca was earlier reported as caused by Xanthomonas arecae. Additional strains of the pathogen were isolated, and detailed biochemical studies were conducted and reported for the first time here, and the properties have been established. The GC content of the DNA of the organism was found to be 59 to 61 percent (mol%).

Studies on the symptomatology of the disease showed that the malady is purely parenchymatous in nature and causes water-soaked chlorotic lesions resulting in blighting of the leaflets. This indirectly leads to a reduction in yield or mortality of the palm depending upon the stage of infection and severity of the disease. The lesions showed copious gummy exudation on the undersurface, which is a characteristic feature of the disease.

Susceptibility to the disease varies significantly with the age of the palm and the age of the leaf. Palms of younger age groups (i.e., between three to five years) were found to be highly susceptible to the disease compared to older palms. As age advanced, susceptibility to the disease also varied, both with the age of the palm as well as with the age of the leaf.

Arecanut varieties tested showed differential reaction to disease development. Actinorhytis calappa, a wild palm related to Areca, was found to be resistant to the disease. The causal bacterium could infect five other economically important crops and three of the ornamental palm varieties tested. It could not infect 13 other plant hosts screened.

The disease incidence has been recorded for a period of five years and has been correlated with environmental factors like maximum temperature, minimum temperature, rainfall, etc. Maximum disease occurs during the active monsoon months (August to October), and thereafter there is a decline in disease incidence. Temperature above 30°C and below 17°C was found to slacken disease incidence and spread.

The causal organism is known to disappear at a faster rate in soil under natural conditions than in sterile soil. None of the weed species tested provided evidence that they act as alternate hosts for the survival of the organism during the off-season to act as the initial inoculum for the development and spread of the disease during the conducive months. The periphery or edges of the blighted portions of the leaves on the palm contained cells which may serve as initial inoculum for the development of the disease during the monsoon months.

The influence of different agronomic practices like spacing of palms, irrigation, fertilization, etc., on the disease development and spread has been investigated. Closer spacing was found to be congenial for disease development and spread. Frequent irrigations (once in 5 to 10 days) were found to aggravate the disease incidence and spread. Higher levels of nitrogen and green leaf manure were found to favor disease development. Phosphorus and potash did not seem to have any influence. Inter-cropping with banana was found to aggravate disease intensity and was detrimental to arecanut plantations.

The organism is known to elaborate a polysaccharide exotoxin. The toxin was found to be a heteropolymer made of glucose, galactose, mannose, and glucuronic acid. The toxin was non-specific and had a wilting effect on tomato cuttings and detached arecanut leaves under in vitro conditions.

The various phenolic fractions in healthy arecanut leaves have been analyzed and were found to consist of fourteen phenolic compounds. The diseased leaf showed another (fifteenth) extra substance formed as a result of host reaction.

Methods of chemical control of the disease have been investigated. Some of the bactericides tested did not have any inhibitory effect on the causal bacterium. Antibiotics like streptomycin sulphate, streptocycline, Agrimycin-100, and tetracycline and its formulations showed good bactericidal effect under in vitro conditions. Tetracycline as prophylactic treatment at 500 mg/ml concentration showed good control over disease development and spread in in vivo tests conducted. It was even found to suppress lesion expansion rate when used as a curative treatment. Stem injection of the antibiotic chemicals (just below the crown) was found to have a longer residual effect than the traditional method of foliar spray application.

The continuous use of antibiotics for disease control has the inherent danger of the development of resistant strains. It is essential to develop alternate control strategies. Use of a mixture of antibiotics instead of a single one will reduce the probability of occurrence of resistant mutants.