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