Thursday, May 8, 2014

Diseases of Apple: White Root Rot Pre-disposing Factors

This is in continuation to my previous post on Diseases of apple: White root rot. In today’s post we are going to discuss about the pre-disposing factors for the disease development.

Pre-disposing factors:

Soil moisture is the major key factor for the proliferation of the disease as the fungus is of polyphagous. The mycelium could grow in soils with 6-89.2 percent water content, but the fungal mats developed most vigorously at 70 percent. Soil type influences greatly the period of survival and spread of soil-borne fungi. The occurrence of white root rot disease of apple trees was closely related to soil conditions. In alluvial areas and regions with non-volcanic ash soils, the pathogen was associated with sandy soils over gravel where available water was the limiting factor. In upland regions, the disease was prevalent on shallow soils overlying stony sub-soils. Of the several factors examined in Himachal Pradesh for their possible role in the longevity and survival of the root rot pathogen, clay loam soil was reported better than clay soil for its longer survival at all temperature ranges and soil moisture regimes. Temperature has been known to play an important role in the survival and spread of the pathogen, 17oC is an optimum temperature level. Of the two factors i.e. temperature and soil moisture, the latter plays more decisive role in increasing the longevity of the pathogen than the former.

The prevalence of white root rot in Himachal Pradesh is limited to high hill soil texture varies from loam to clay loam. The soil reaction is slightly acidic and this zone is ideally suited for apple cultivation. It receives an average of 100-140 cm rainfall per annum. The rainy season in the apple zone is during June to September, when also a higher incidence and severe symptoms of root rot are observed, indicating that the disease spread and severity are related to excessive soil moisture regimes at higher temperature. The higher elevation range was observed favourable for the spread of D. necatrix being high rainfall area and low temperature conditions prevailing for long durations. Besides soil, pH also seems to have a strong relationship with the survival of the pathogen flourishing best in 6- 6.5 pH range. Incidentally the larger area under apple cultivation in Himachal Pradesh also falls within this pH range.

Physico-environmental conditions viz., soil type (loam and clay laom), soil reaction (pH6-6.5), soil moisture (>70%) and Temperature (17oC) play an important role in incidence and survival of Dematophora necatrix.

In coming post we shall be discussing about the management of the white root rot disease.