In this article we will discuss about the sandal spike disease caused by mycoplasma.
Subject-Matter of Sandal Spike Disease:
The disease was first reported from Coorg distinct of Karnataka State in 1903 by Barban. The disease was first thought to be caused by Viruses but in 1969, Varma et al reported that Mycoplasma like organism is the causal organism.
The disease is one of the most destructive diseases of Sandal tree – an economically important tree and is prevalent in the areas wherever Sandal trees are grown. Significantly the disease is confined to India and Indonesia.
In India, Sandal tree to common in Southern States including Karnataka, Kerala and Tamil Nadu. According to Iyengar (1969), an annual loss amounting to Rs. 3 million has been estimated due to this disease.
Symptoms of Sandal Spike Disease:
Two types of symptoms are produced. The common symptom called the ‘rosette spike’ is characterised by severe reduction in leaf size and reduction of internodes. This results in crowding of leaves on leaf bearing branches.
The new leaves are further reduced in size. Such leaves become stiff and stand out as spikes. In advanced stages of the disease, the leaves become reddish or yellowish. This symptom appears just before death of the tree.
The flowers of the infected trees become leaf like and the infected plants do not bear any fruits, the ends of roots of infected plants die out resulting in the loss of the haustorial connection with the host plants. Ultimately the infected plants drie up and die.
The other symptom called ‘pendulous spike’ is due to continuous apical growth of individual shoots without proper thickening and results in drooping of shoots. The dormant buds do not grow and no rosettes are produced. In this symptom, the roots and haustorial connection are not damaged.
Causal Organism:
MLO ( Mycoplasma like organisms) is the causal organism. MLO size in the host ranges from 60-750 nm. The common form is ellipsoid, measuring 180-220 x 250-300 nm.
The MLO is reported to infect a large number of plant species such as Eucalyptus grandis, Vinca rosea, Zizyphus oenoplea, Dodonea viscosa.
Disease Cycle:
The disease is transmitted through root contacts and insect vectors. The disease can also be transmitted through Vinca rosea and vice versa through dodder. The most common insect Vectors are Moonia alhimaculata and Nephotettix virescence.
Control Measures of Sandal Spike Disease:
No specific method to control the disease has so far been developed. However, it is claimed that planting of Mysore gum trees (ahybriod of Eucalyptus tereticornis) at a distance of 10-20 metres from the Sandal trees keeps the latter free of infection.
As with other mycoplasmal diseases, treatment with tetracycline checks the disease. Attempts aie also being made to develop disease resistant trees via genetic engineering and tissue culture technique.