FOREST DISEASE MANAGEMENT

DISEASES OF NEEM (Azadirachta indica)
            Earlier reports show that a few fungi are known to cause diseases in nurseries and plantations of neem. They are fungi like species of Alternaria, Cercospora, Colletotrichum, Fusarium, Oidium, Ganoderma and Corticium.  A brief description of the various diseases and their control measures in nursery is given below:

(a) Damping off:
            Among the nursery diseases, damping-off is the most prevalent and highly destructive disease and cause heavy loss of seedlings. It is referred to a group of disease namely pre-emergence and post-emergence damping off depending on the stage of growth of seedlings when they are attacked. This disease is caused by various of soil fungi such as Pythium, Phytophthora, Fusarium and Rhizoctonia of which the last two fungi are quiet prevalent in forest nurseries in India. 
High soil temperature, excessive soil moisture, high soil pH (alkaline), high nitrogen content, low light intensity due to shading, stiffy or clayey soil with poor drainage, dense sowing are the conditions which favour the disease development in serious proportion. In case of Neem seedlings, the disease was caused by Fusarium oxysporum (Mehrotra and Pandey, 1992).

Control Measures:
The disease has been managed through: (a) cultural practices aimed at favouring plant growth and discouraging the growth of plant pathogen; (b) use of chemicals like formalin and suitable fungicides. Formalin is applied as soil fumigant and the fungicides can be applied either as soil drench or soil mix.  Also, seed dressing with fungicide (Bavistin) in certain cases found very effective. Potting media should be properly sterilized by solar heating before use.

(b) Leaf Web Blight:
            It is caused by Rhizoctonia solani.  The disease appears in the nursery after the regular monsoon rains set in (Mehrotra, 1990; Sankaran et al., 1986).
Symptoms:
            Development of greyish brown blotches which increase in size with the advancing fungal hyphae and ultimately engulf the entire leaf blade. The infected adjoining leaves get joined together by the fungal hyphae as if caught in a spider’s web, hence the name web blight. The leaflets or the entire pinnae become detached prematurely. The disease spreads through contact of the overlapping foliage.

Control Measures:
            The disease has been managed through interacted approach which includes measures such as sanitation and cultural practices. Application of fungicide (Bavistin 0.1% a.i.) is found effective.

(c) Colletotrichum Leaf spot and blight:
            It is caused by Colletotrichum gloeosporioides. It has been recorded in a serious form at New Forest, Dehra Dun. It appears in nursery at the end of the September or first week of October (Mehrotra and Pandey, 1992).
Symptoms:
            The fungus causes leaf spots which increase rapidly in size covering large leaf areas. The infected leaves present a blighted appearance and are eventually shed. Severely infected seedlings show premature defoliation.

Control Measures:
            Application of Blitox fungicide (0.2% a.i.) twice at weekly intervals is found effective in controlling the disease.

(d) Alternaria Leaf spot and blight:
It is caused by Alternaria alternata. It is a destructive pathogen. It appears late in the growing season in the last week of October or early November at New Forest, Dehra Dun (Mehrotra and Pandey, 1992).  It attacks the leaves when the leaves become old and contain less soluble sugars.

Control Measures:
Application of Blitox fungicide (0.2%) at fortnightly intervals is found very effective.

(e) Pseudocercospora Leaf spot:
It is caused by Pseudocercospora subsessilis. The disease occurs throughout the natural distribution of neem.
Symptoms:
            The infection spots are brown in colour interspersed with white patches. The fungus sporulates on the under surface of the leaf and produces conidia which appear grayish in mass. The heavily infected leaves turn pale and are shed prematurely.

Control Measures:
            Application of Mancozeb in combination with Brestan is found effective in controlling the disease.

(f) Powdery Mildew:
            This disease is caused by Oidium azadirachtae. 
Symptoms:
White patches seen on the surface of the leaves. These patches coalesced and covered the whole leaf lamina giving greyish white appearance. Severely infected leaves and leaflets defoliated prematurely.

Control Measures:
Foliar spray of Bavistin fungicidal solution (0.01%) is found to be effective in minimizing the disease.

(g) Other Foliar Diseases:
Bacterial leaf spot:
Bacterial leaf spot is caused by a bacterium, Xanthomonas azadirachtii and Pseudomonas viticola.
Leaf spot and Blight:
Leaf spot is caused by Colletotrichum capsici. Leaf Blight and stem rot are caused by Sclerotium rolfsii.
Seedling wilt:
Seedling wilt is caused by Fusarium solani. Twig canker and shot hole in leaves are caused Phoma sp.