Diseased Tomato Plants ??

Can Anyone Identify what disease is this and how can we control it ?


The leaves of the Tomato crop have spots, which are small, circular to angular, isolated, scattered and pale brown to black in color. These spots seem to be surrounded by a narrow chlorotic zone in some cases. Some of them also have brown lesions near the stem base. A few plants are completely defoliated. There is a picture below showing the situation.

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I posted about this article in various social networking sites with an urge to discover more about this disease. I found a good amount of knowledge of the disease and its control measures.

TomatoDrawWlt

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EARLY BLIGHT OF TOMATO

Introduction

Early blight of tomato, caused by the fungus Alternaria solani, is perhaps the most common foliar disease of tomatoes in the Northeast and is also common on potatoes. This disease causes direct losses by the infection of fruits and indirect losses by reducing plant vigor. Fruit from defoliated plants are also subject to sunscald.

Environmental Conditions

Alternaria spores germinate within 2 hours over a wide range of temperatures but at 80 to 85ºF may only take 1/2 hour. Another 3 to 12 hours are required for the fungus to penetrate the plant depending on temperature. After penetration, lesions may form within 2-3 days or the infection can remain dormant awaiting proper conditions (60ºF and extended periods of wetness). Alternaria sporulates best at about 80ºF when abundant moisture (as provided by rain, mist, fog, dew, irrigation) is present. Infections are most prevalent on poorly nourished or otherwise stressed plants.

Symptoms

Early blight produces a wide range of symptoms at all stages of plant growth. It can cause damping-off, collar rot, stem cankers, leaf blight, and fruit rot. The classic symptoms occur on the leaves where circular lesions up to 1/2″ in diameter are produced. Within these lesions dark, concentric circles can be seen. The leaf blight phase usually begins on the lower, older leaves and progresses up the plant. Infected leaves eventually wither, die, and fall from the plant.

Survival and Dispersal

The fungus spends the winter in infected plant debris in or on the soil where it can survive at least one and perhaps several years. It can also be seed borne. New spores are produced the following season. The spores are transported by water, wind, insects, other animals including man, and machinery. Once the initial infections have occurred, they become the most important source of new spore production and are responsible for rapid disease spread.

Control

  1. Use only clean seed saved from disease-free plants.
  2. 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.
  3. Practice crop rotation to non-susceptible crops (3 years). Be sure to control volunteers and susceptible weeds.
  4. Promote good air circulation by proper spacing of plants.
  5. Orient rows in the direction of prevailing winds, avoid shaded areas, and avoid wind barriers.
  6. Irrigate early in the day to promote rapid drying of foliage.
  7. Healthy plants with adequate nutrition are less susceptible to the disease.
  8. Minimize plant injury and the spread of spores by controlling insect feeding.
  9. 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.
  10. Use resistant or tolerant varieties.
  11. The preventative fungicide chlorothalonil (Bravo) used on a seven to ten day schedule gives effective control.
Fungicides for Early Blight Control
Fungicide Typical Application Interval Examples of Trade Names
azoxystrobin, pyraclostrobin 7 to 14 days Quadris, Amistar, Cabrio EG
Bacillus subtilis 5 to 7 days Seranade
chlorothalonil 7 to 14 days Daconil, Bravo, Echo, Fungonil and others
copper products 7 to 14 days Bordeaux Mixture, Kocide, Tenn-Cop,Liqui-cop,  Basicop, Camelot
hydrogen dioxide Commercial only, see label Oxidate
mancozeb and maneb 7 to 14 days Dithane, Penncozeb, Manex, Mancozeb, Maneb
potassium bicarbonate 5-14 days as needed Armicarb 100, Firststep
ziram 7 to 14 days Ziram

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