EFFECT OF TEMPERATURE ON Alternaria alternata CAUSING BROWN SPOT OF POTATO

2018) Effect of


Introduction
The genus Alternaria contains a diverse and ubiquitous population of fungi, including aggressive and opportunistic plant pathogens. Of the two best known and economically important members of the genus one is A. solani, the causal agent of early blight on potato, the most infamous one and the other is Alternaria alternata (Fr.) Keissler which was better known for its saprophytic nature. However, over the time scale, it has bagged pathogenic status by its virtue of producing toxins in crops like Tomato, Pepper, Mandarins, Bean, Tobacco, Durum Wheat, Mango, Pistachio, Pears, Citrus, Almond and Persimmon etc [1,2]. Quite recently A.alternata has been reported to cause brown necrotic lesions on potato foliage and black pit disease on tubers from Israel [3], Brazil [4], North America [5], South Africa [6], Pakistan [7] and several other parts of the world. Losses due to the disease have typically been estimated to around 20 percent. However, there have been instances of losses amounting to 70-80 per cent, when the disease was left uncontrolled [8]. The occurrence of this disease was recently observed in several parts of Eastern Uttar Pradesh. In order to understand the physiological variability of the pathogen a study was conducted to know the effect of different temperatures on the growth characteristics. Isolation and maintenance of isolates: Leaf sample with typical brown spot symptoms were excised from the collected potato samples and cleaned under tap water to remove dust particle. The excised tissue was cut into small bits and sterilized in 0.1% sodium hypochlorite for 30 seconds under aseptic condition. These bits were than washed thrice in sterile distilled water for 15 seconds and placed in sterilized blotting paper to remove excess moisture. These bits were placed aseptically in Petri dish containing solidified Potato Dextrose Agar (PDA) medium. These inoculated plates were placed in BOD incubator for growth of the organism at 26±2°C. On the basis of morphology of conidia and conidial chains the pathogen was identified as A. alternata and purified by single spore isolation method. After observing growth of fungus sub-culturing was done in another Petri dish these purified cultures were then maintained for further studies in slants by sub-culturing and will be preserved under refrigerated condition (at 5°C).

Identification of the pathogen:
The pathogen was identified on the basis of its cultural & morphological characteristics as well as pathogenicity test. Slides from cultured mycelia were prepared on lacto phenol and observed under compound microscope. Identification of the pathogen was done on the basis of cultural and morphological grounds as described by various authors [6,[9][10][11][12][13] Evaluation method: Growth of different isolates of A. alternata was studied in four different temperatures viz., 15, 20, 25and 30 0 C on potato dextrose agar medium (PDA). Petri plates were poured with about fifteen to twenty millilitres sterilized molten medium aseptically. Each isolate was replicated twice under each temperature to be studied. After solidification of the medium each plate was centrally inoculated with 10 mm mycelial disc from the margin of 8 days old culture (maintained at 26+ 1ºC on PDA) with the help of a sterilized cork borer. The colony diameter was measured eight days after inoculation. Also, the rate of growth of each isolate was assessed by measuring radial mycelial growth of each isolate daily. Where, G R = Growth rate (mm hr -1 ), S = Colony diameter (mm), T = Time (days.)

Statistical analysis:
The data recorded during course of investigations were subjected to statistical analysis using STPR software. The significance of treatment difference was tested by F-test on the basis of null hypothesis. The appropriate standard error (S.Em±) was computed in each case. Coefficient of variance per cent was also worked out for all the characters.

Result and Discussion
The identity of the fungi causing brown spot disease was confirmed as A.alternata on the basis of its cultural and morphological features. The recorded characteristic of all the thirty isolates employed in the current study was in accordance with the literature [9][10][11].

Conclusion
The present study can be concluded that, A. alternata causing brown spot can be grown at 25 -30°C for the maximum radial growth. The variability in growth rate indicates the ability of the pathogen to adjust to various temperatures.

Application of research:
The results of study indicate that the pathogen can survrive in a wide range of temperature and hence the results of this study can be used further by the researchers for characterisation of pathogen and to design efficient management strategy.

Research Category: Plant Pathology
Abbreviations: PDA: Potato Dextrose Agar