Molecular Plant Breeding 2016, Vol.7, No.25, 1
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wetness duration and relative humidity (moisture) affect the development of early blight on potatoes (Adams and
Stevenson, 1990, Vloutoglou and Kalogerakis, 2000). Temperature increases
A. solani
infection and sporulation
(Vloutoglou and Kalogerakis, 2000). Water in the form of high relative humidity, rainfall or dew accumulation can
increase conidia germination and pathogen infection (Rotem, 2004). Alternating low and high humidity conditions
have also been shown to favour disease development (Van der Walls et al., 2001). Early blight is also enhanced
through continuous potato production (Olanya et al., 2009). Symptoms are initially observed on older, senescing
leaves (Rands, 1917, Pscheidt, 1985; Shuman, 1995). Characteristic symptoms are dark brown or black lesions
with concentric rings on leaves, which produce a 'target spot' effect. Enlarging lesions are often surrounded by a
narrow chlorotic halo. Lesions are similar on all hosts (Pscheidt, 1985).
Alternaria solani
over winters as mycelium or conidia in plant debris, soil, infected tubers or on other host plants
of the same family (Pscheidt, 1985; Pelletier, 1988; Shuman, 1995). The primary inoculums, conidia, are
produced in the spring, and are then splash or wind dispersed to the lower leaves of the plant. Spore germination is
facilitated by free moisture, but can be induced by relative humidity close to saturation. Germ-tubes form
appressoria, and penetrate the epidermis directly or through wounds or stomata. The minimum temperature for
infection can be as low as 10°C, the maximum >35°C, and the optimum between 20°C and 30°C. Incubation
periods vary greatly, depending on age and susceptibility of plants (Rotem, 1994). Sporulation occurs between
5°C and 30°C, with the optimum around 20°C (Pscheidt, 1985).
A. solani
is one of the few pathogens that is able
to overcome a lack of prolonged humidity, by using several short wet periods (usually at night) interrupted by dry
intervals during the day, otherwise known as interrupted wetting periods (Bashi and Rotem, 1975, Rotem et al. ,
1978). This adaptation allows
A. solani
to thrive equally well in areas with continuous humidity and in areas with
alternating wet-dry conditions. Epidemics do not generally occur until late in the season, when the plants are most
susceptible.
Under temperate Kashmir conditions, early blight of potato caused by
Alternaria solani
is posing a great threat for
its cultivation. The systemic study on potato early blight has not been conducted so far under Kashmir conditions.
Therefore keeping in view the devastating nature of disease a detailed investigation was undertaken to study the
perpetuation of the causal organism.
1 Result and Analysis
The perpetuation of the fungus
Alternaria solani
was studied on diseased leaves by placing the leaves on ground
surface and at 20 cm depth, as well as on diseased potatoes kept in ambient store during the year 2009 and 2010.
1.1 Survival on over-wintered diseased leaves
Diseased leaves in mesh wire bags were placed on ground surface as well as buried at 20 cm depth. Observations
regarding the spore production and their viability were recorded at fortnightly intervals and are presented in Table
1, Table 2.
Perusal of data revealed that the spore production in over-wintered diseased leaves continued for comparatively
longer period upto first fortnight of June in both the years (2009 and 2010) of experimentation, when kept at
ground surface. On diseased leaves kept at 20 cm deep in soil, the spores were altogether absent throughout the
observation period, because the leaves were decomposed. The average number of spores cm
-2
diseased leaf area
increased upto first fortnight of June in the year 2009 and 2010, with maximum number of 294 and 323 spores,
respectively. However, by the first fortnight of July, the number gradually declined to 160 and 220 spores,
respectively.
The viability of spores exhibited a sharp decline with the increase in depth of placement. Highest spores viability
was recorded in over-wintered diseased leaves at ground surface. The highest spore viability of 44.3 and 49.3 per
cent in leaves on ground surface was recorded in the first fortnight of June, 2009 and 2010, respectively. However,
in leaves buried at 20 cm depth the spores were altogether absent because the leaves were decomposed.
