AgMOOCs  Assignment on Use Social Networks to find solutions

Based on the suggestions received from my friends( facebook), I concluded that the picture shows early blight in tomato. 

Detailed Report on

Early Blight or Alternaria Leaf Spot 

PATHOGEN: Alternaria solani

SYMPTOMS:

Foliar symptoms generally occur on the oldest leaves and start as small, brownish to black lesions. These leaf spots enlarge up to ½ inch (1.3 cm) in diameter in a characteristically concentric fashion. The area around the spot may become yellow, as may entire severely affected leaves. Under favorable conditions, significant defoliation of lower leaves may occur, leading to sunscald of fruit.

Green or red fruit may be infected by the fungus which invades at the point of attachment between the stem and fruit, and through growth cracks and wounds made by insects. Dark lesions enlarge in a concentric fashion and may affect large areas of the fruit. Mature lesions in fruit are typically covered by a black velvety mass of fungal spores. Stem lesions are dark, slightly sunken and enlarge concentrically. Basal girdling and death of seedlings may occur.

DISEASE CYCLE & EPIDEMIOLOGY:

Alternaria solani overwinters primarily on infected crop debris. The dark pigmentation of the mycelium increases resistance to lysis which extends the survival time in the soil to several years. Thick-walled chlamydospores have been reported, but they are found infrequently. In mild climates the pathogen can survive from season to season on volunteer tomato and potato plants as well as other weedy Solanaceous hosts such as horsenettle and nightshade.

Warm, humid (24-29°C/ 75-84°F) environmental conditions are conducive to infection. In the presence of free moisture and at an optimum of 28-30°C (82-86°F), conidia will germinate in approximately 40 min. Desiccated germ tubes are able to renew growth when re-wetted, and, hence, infection can occur under conditions of alternating wet and dry periods. Germ tubes penetrate the leaf epidermis directly or enter through stomata. Infection of potato tubers usually occurs through wounds in the tuber skin inflicted during harvest. Wet conditions at harvest provide a favorable environment for spore germination as well as causing swollen lenticels on the tubers which are easily invaded.

Time from initial infection to appearance of foliar symptoms is dependent on environmental conditions, leaf age, and cultivar susceptibility. Early blight is principally a disease of aging plant tissue. Lesions generally appear quickly under warm, moist conditions on older foliage and are usually visible within 5-7 days after infection.   A long wet period is required for sporulation but it can also occur under conditions of alternating wet and dry periods.  Conidiophores are produced during wet nights and  the following day light and dryness induce them to produce spores, which emerge on the second wet night.  Secondary spread of the disease results from conidia being dispersed mainly be wind and occasionally by splashing rain or overhead irrigation.  Early blight is considered polycyclic with repeating cycles of new infection.  This is the period when the disease has the potential to spread rapidly and build up to damaging levels in the crop.

CONTROL MEASURES

Cultural Controls:  

1.    Use pathogen-free seed and transplants.

2.  Maintain plant vigor through adequate irrigation and fertilization to increase disease resistance.

3.    Avoid plant injury which allows entry of the pathogen and spread of the fungus through adequate insect management

4. Remove and destroy crop residue at the end of the season. Where this is not practical, plow residue into the soil to promote breakdown by soil microorganisms and to physically remove the spore source from the soil surface.

5.  Practice crop rotation to non-susceptible crops (3 years). Be sure to control volunteers and susceptible weeds.

6.    Promote good air circulation by proper spacing of plants.

7.    Orient rows in the direction of prevailing winds, avoid shaded areas, and avoid wind barriers.

8.    Hand picking diseased foliage may slow the rate of disease spread but should not be relied on for control. Do not work in a wet garden.

9.    Use resistant or tolerant varieties.

Chemical control:

1. The preventative fungicide chlorothalonil (Bravo) used on a seven to ten day schedule gives effective control.
2. Using a good broad spectrum fungicide, such as GreenCure and spraying every 2 weeks during periods of optimum conditions for the Early Blight disease.