In this article we will discuss about the tikka disease of groundnut caused by deuteromycetes.
Host: Arachis Hypogea L
Pathogen: Cercospora sp
Introduction to the Tikka Diseases of Groundnut:
One of the best known leaf spot diseases is that of Arachis hypogea L. (groundnut). Popularly it is called the tikka disease. The tikka disease is a serious disease occurring in areas where the groundnut crop is grown in India.
The spots appear on the host leaves when the plants are one or two months old. Later necrotic lesions appear on the stem as well. In fact there are two leaf spot diseases of groundnut caused by two different species of form genus Cercospora namely, C. arachidicola.
The leaf spot disease caused by the former is more common, dangerous and does greater damage than the latter. The spot produced by C. personata are numerous. These weaken the host plant and lead to defoliation which adversely affects size and quality of the fruit.
The latter remains small in size and fewer in number. Sundaram (1965) reported that in the case of severe infection the crop yield may be reduced to 22%.t
Causal Organism:
Tikka disease of groundnuts is caused by two species of Cercospora: Cercospora personata (Berk. & Curt.) Elle and Eve., now known as Cercosporidium personatum (Berk. & Curt.) Deighton and Cercospora arachidicola Hori. The two form-species differ from each other with respect to the size, shape and colour of necrotic lesions they produce, conidia formation and the nature of the mycelium.
These differences are summed up in the Table given below:
TABLE:
C. personata (Fig. 22.24)
Symptoms of Tikka Disease:
1. Leaf spots are small, more circular and about 1-6 mm in diameter (A).
2. The necrotic lesions appear on both the leaf surfaces and change from dark brown to dark in colour.
3. Young spots lack bright yellow halo which, however, develops around the older ones.
Mycelium of Tikka Disease:
4. The mycelium consists of hyphae which are entirely internal.
5. The septate hyphae ramify in the intercellular spaces and obtain nutrition by sending branched haustoria into the mesophyll (both spongy and palisade) cells.
6. The unbranched conidiophores arise in tufts from a dense, globular, and brown to black stroma (23-30 µ in dia.) and emerge by rupturing the host epidermis (B).
7. They are geniculate, olivaceous brown, 24-54 p long and 5-8 µ broad and are either aseptate or septate with 1-2 septa.
8. The conidiophores are confined to the lower surface of the host leaf and are seen arranged in concentric circles in the tuft.
9. The conidia (C) are obclavate or cylindrical, light coloured, 18-60 µ in length and 6-11 µ in breadth. They are septate with 1-7 septa and have bluntly rounded ends.
C. arachidicola:
1. The leaf spots are comparatively larger in size, irregularly circular in outline and 4-10 mm in diameter.
2. The leaf spots are often confluent and necrotic lesions occur on both the surfaces. Those on the upper surface are reddish brown to black and on the lower surface are light brown.
3. There is a yellow halo around each spot but halos on the lower surface are less distinct.
4. It consists of both external and internal hyphae.
5. The internal hyphae are both intercellular and intracellular. The haustoria are absent.
6. The unbranched conidiophores arise scattered from a deep brown stroma (25-100 p in dia.).
7. They are geniculate, yellowish brown and 15 to 45 p long and 3-5 p broad and are either aseptate or septate with 1 to 2 septa.
8. The conidiophores usually occur on the upper surface of the host leaf but occasionally are found on the lower surface as well and thus described as amphigenous. They are sparse and do not occur.
9. The conidia are obclavate hyaline or pale yellow to slightly olivaceous, 38-108 µ long and 8-6 µ broad. They are septate with 4-12 septa and have rounded to distinctly truncate base and sub-acute apex.
Disease Cycle:
Primary infection takes place by soil borne conidia, which in the soil and on seeds in the shells. The spread of the disease during the season is by means of wind disseminated conidia.
Penetration is direct through the epidermis and also by way of stomata. Chiefly it occurs through the upper epidermis. However, there is possibility of infection through both the leaf surfaces.
Predisposing Factors of Tikka Disease:
According to Sulaiman and Agashe (1965), maximum predisposing factors for the development of disease prevail in the month of September. Prolonged low temperature and heavy dew favour severe infection.
According to Shanta (1960) and Jensen and Boyle (1965), relative humidity plays an important role in infection. Ramakrishnan and Apparao (1965) reported that a period of 3 days of high humidity is essential for maximum infection.
Shanta studied the effect of NPK fertilizers on the incidence of disease and found that the application of nitrogen and phosphatic fertilizers increase disease incidence whereas potash decreases it slightly.
Ravindar Nath, Kulkami and Appaji Rao (1965) who carried on preliminary biochemical observations of Tikka disease, have suggested that ascorbic acid and riboflavin content of host leaves appear to play some part in susceptibility and resistance to the disease.
According to them, increase in riboflavin content of the leaves is correlated with resistance whereas increase in ascorbic acid is directly related to susceptibility.
Control Measures of Tikka Disease:
Rotation of crops, seed treatment and disposal of infected host debris by burning or burying in deep pits eliminate chances of primary infection from the soil borne inoculum. The seeds within the shells are disinfected with sulphuric acid.
Without shells, they are soaked for half an hour in 0.5% copper sulphate solution. Agrosan GN dressing of naked seeds is equally effective.
To check secondary spread of disease in the field, spraying with suitable fungicides is the only remedy. Among the fungicides the use of Bordeaux mixture has given good results.
Addy and Dash (1967) recommended Dithane while Shanta (1960) considered red oxide of copper plus sulphur as highly effective. Sulaiman (1964) preferred copper dust (300 mesh) plus sulphur in the ratio of 1: 1. Harrison (1973) found that Daconil and Benlate are the two more effective fungicides than Dithane M45 for pea nut leaf spot disease control.
The sprays of these fungicides increased yield as much as 100%. Early maturing varieties are reported to possess resistance to the disease.