Daily Archives: March 7, 2012

Bacterial Blight

                    Bacterial Blight

   The Systematic name of Xanthomonas malvacearum

* Genus: Xanthomonas
* Family: Xanthomonadaceae
* Order: Xanthomonadales
* Class: Gammaproteobacteria

          About Bacterial Blight

                    Bacterial blight (Xanthomonas malvacearum) is favoured by wet windy weather and injured tissues are very susceptible to infection. Disease symptoms are favoured when temperatures rise above 25oC and relative humidity exceeds 85 per cent. Hail-damaged crops are often severely infected. The use of acid delinted seed will provide good control of seed-borne bacteria. Varieties with some resistance to this disease are available and could be useful in areas where the disease is frequently severe. Prompt cultivation of crops after harvest, so residues have ample time to decompose, will also assist in the control of the disease.

                                Bacterial Blight
                                              Fig1.Cotton bacterial blight.

Symptopms of Fusarium Wilt

                              Bacterial Blight
                             Fig 2.Boll rot caused by bacterial blight at the base of a boll.

                              Bacterial Blight
                             Fig 3.Angular leaf spot caused by Xanthomonas campestris pv.

                   On cotyledons, bacterial blight causes small, quarter-inch-diameter lesions that are initially dark green turning to dark brown. It may cause black cankers on the stems of seedlings and older plants. Symptoms on the leaves may appear throughout the growing season. Symptoms will be most severe on leaves when the humidity is high with air temperatures average 86 to 95 degrees Fahrenheit and when rain, heavy dew, fog or sprinkle irrigation are frequent. The first leaf symptoms are light-green spots that are visible on the upper and lower surfaces of the leaves. These spots are usually 1/16 inch to 1/8 inch in diameter and quickly turn dark brown to black .The margin of these spots will have some sharp angles. Leaves with lots of individual lesions may defoliate prematurely. This disease may also develop on bolls. The round, light-green lesions on bolls may develop into boll rot.

      Management of Bacterial Blight

                              Bacterial Blight
                             Fig 4.Boll rot caused by bacterial blight at the base of a boll.

                   Remedial actions to arrest disease development are not available. The only positive action growers can take is to use growth regulators to prevent rank growth. Keeping the canopy as open as possible to reduce humidity and promote drying of the foliage may prove beneficial in limiting the progress of this disease. Do not cultivate or move equipment through fields when foliage is wet. Infested fields should be harvested as soon as possible. The cotton variety, seed lot and a rating as to disease severity should be determined. Stalks should be shredded at the first opportunity. Fields that have bacterial blight this year should be planted to a blight-resistant variety next year or rotated to a different crop.

      Preventive Measures for Bacterial Blight

                              Bacterial Blight
                                      Fig 5.Bacterial blight infestated leaf

I. Plant high-quality, disease free, acid delinted seed.
II. Plant blight-resistant varieties if available.
III.Scout fields and identify infected plants and varieties.
IV. Shred stalks and incorporate cotton debris.
V. Do not cultivate or move equipment through fields when foliage is wet.

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Verticillium Wilt

                    Verticillium Wilt

   The Taxonomy of Verticillium wilt

* Genus: Verticillium
* Family: Incertae sedis
* Order: Hypocreales
* Class: Sordariomycetes

   The Taxonomy of Verticillium dahliae

* Kingdom:Fungi
* Phylum: Incertae sedis
* class:   Hypocreales
* order:   Hypocreales

* Genus:  Verticillium.

          About Verticillium Wilt

                    The Verticillium dahliae fungus is a common soil inhabitant, which can infect many crops and weeds. It penetrates the roots and grows in the water conducting tissue, blocking water uptake and causing wilt symptoms.
The disease is favoured by cooler temperatures, excessive soil moisture and excessive soil nitrogen levels. Resistance to the disease is temperature sensitive. Varieties that are resistant at 25-27°C are susceptible at 20-22°C. Proper management of irrigation and fertiliser application should provide some measure of control of this disease, though the selection of resistant varieties is the major management strategy.
Cotton should not be planted in areas where there is a high infestation of the fungus in the soil, and individual fields should not be sown to cotton for more than four years in succession. Susceptible weeds in cotton fields should be destroyed.
Yield is most significantly affected when weather conditions are conducive to severe symptoms mid-season and plants do not have time to compensate. Under such conditions, yield reductions of up to 30 per cent are possible.

                              Verticillium wilt
                                    Fig 1.Symptoms of Verticillium Wilt.


      Symptopms of Verticillium Wilt

                              Verticillium wilt
                             Fig 2.Symptoms of wilting are most evident on warm.

                   Although Verticillium wilt can attack seedlings, symptoms of this disease are most often observed on plants from first flower to harvest. There is a progressive yellowing and dying of the leaf tissue between the major veins of the leaves, beginning on the lower leaves and then progressing to the younger leaves. These symptoms are similar for Verticillium and Fusarium wilt.
Affected plants may shed both leaves and bolls prematurely, and the inside of the lower stem will be streaked with brown .This discoloration is usually evenly distributed throughout the stalk. Most plants will survive throughout the growing season and will put on new growth at the base of the plant..

      Management of Verticillium Wilt

                              Verticillium wilt
                                Fig 3. Verticillium wilt of cotton diffuse leaf mottle

* Plant tolerant varieties.
* Do not use excessive nitrogen.
* Rotate cotton with corn and grasses (wheat, oats, etc.).

Fusarium Wilt

Fusarium Wilt

The Taxonomy of fusarium wilt

* Genus: Fusarium

* Family: Nectriaceae

* Order: Hypocreales

* Class: Sordariomycetes

About Fusarium Wilt

Fusarium wilt (Fusarium oxysporum, F. vasinfectum) was first discovered in the United States in 1892, in Egypt ten years later. Wet weather conditions (temperature above 23°C and relative humidity exceeding 85%) are particularly conducive to disease development. Disease incidence can be higher in plants with injured tissues (for example, plants damaged by nematodes). Plants can be affected by the disease at any stage during the season. The vascular tissue of infected plants exhibits a brown/chocolate discolouration through the main stem. Infected water-conducting stem tissues become inactive, causing wilted foliage. Plant death, wilting, yellowing and defoliation are typical of disease symptoms. Leaves turn yellow between veins and eventually shed to leave bare stems. Once the fungus has colonised the plant (diagnosis is confirmed by splitting the stem to reveal dark brown), it most likely causes the death of the host. There is no commercially viable way to eradicate the disease once established (apart from soil fumigation, which is excessively expensive). The impact of the disease may nonetheless be reduced by the use of varieties with high levels of resistance to Fusarium wilt, or by avoiding crop stresses such as over-irrigation and over-application of nitrogen. Fusarium wilt is now an important constraint to sustainable cotton production.


      Fig1.Brown streaks visible inside cotton stems are a symptom of Fusarium wilt.

Symptopms of Fusarium Wilt

      Fig 2.Brown streaks visible inside cotton stems are a symptom of Fusarium wilt.

Fusarium wilt symptoms can appear at any stage of plant development and will vary with environmental conditions. Generally, the first symptoms appear on leaves about the time of first flowering.Symptoms appear first at the margin of the leaf, where small areas turn yellow and then brown , and the leaf then wilts. Although wilting is usually gradual, sudden wilting may occur shortly after a midsummer rain that follows a dry period. Tissue inside the lower stem of infected plants will be discolored.Although Fusarium wilt can occur in the absence of root-knot nematodes, nematode feeding increases the susceptibility of plants to this disease.When both the Fusarium wilt fungus and nematodes are present, there is a significant increase in the damage to plants.

Management of Fusarium Wilt

              Fusarium wilt
      Fig 3.Fusarium oxysporum on cotton plant.

Symptoms are mostly noticed when plants are eight weeks old. The leaves turn yellow, discoloration starting from margin followed by epinasty and defoliation leaving the stem bare. On diagonally cutting the stem, a dark brown ring can be seen beneath the bark. Hot and dry periods followed by the rains provide conditions for maximum disease development. Heavy clay soil with alkaline reaction is highly favorable for the pathogen. The nematode and ash weevil grubs provide the fungus an easy means of entry to the roots. Cultural practices help in checking the spread of the disease. Apply adequate dose of farm yard manure or other manure, potash with a balanced application of nitrogenous and phosphatic fertilizers. Ammoniacal nitrogen should be avoided as it increases the disease. Give spot drenching with carbendazim 0.1%.

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