Fungal Blight/Rust on Upland Cotton and its Control

Sunday, 20 January 2013

By: Shehzad Ahmad Kang

Introduction

Upland cotton{Gossypium hirsutum (2n = 52}) is cash crop of Pakistan. Majority of industries based on cotton crop, so it is known as back bone of Pak economy. Season 2011-2012 Pakistan produced over 15 million bales (Anonymous, 2011-12). Last year (2011) and current season cotton crop was severely affected by leaf/stem blight or rust. This fungal disease caused by an organism Rhizoctonia solani which primarily attacks seeds of plants below the soil surface, but can also infect bolls, roots, leaves and stems. The most common symptom of Rhizoctonia is “damping off”, or the failure of infected seeds to germinate. Rhizoctonia solani may invade the seed before it has germinated to cause this pre-emergent damping off, or it can kill very young seedlings or mature plants. Seeds that do germinate before being killed by the fungus have reddish-brown lesions and cankers on stems, leaves and roots.

There are various environmental conditions that put the plant at higher risk of infection due to Rhizoctonia, The pathogen prefers warmer wet climates for infection and growth. Post-emergent damping off is a further delay in attack of Rhizoctonia solani. The seedling is most susceptible to disease in its juvenile stage in patches form.

Rhizoctonia solani can also cause hypocotyl and stem cankers on mature plants of cotton. Strands of mycelium and sometimes sclerotia appear on their surfaces. Roots will turn brown and die after a period of time. The fungus has a wide host range. Strains of Rhizoctonia solani may differ in the hosts they are able to infect, the virulence of infection, selectivity for a given host ranging from non-pathogenic to highly virulent, the temperature at which infection occurs, the ability to develop in lower soil levels, the ability to form sclerotia, the growth rate, the survival in a certain area. Rhizoctoniaattack start from zero node (soil surface) to upper nodes, leaves and whole plant.

 

Environment

The pathogen is known to prefer warm wet weather, and out breaks typically occur in the early summer months. Most symptoms of the pathogen do not occur until late summer and thus most farmers do not become aware of the diseased crop until harvest. A combination of environmental factors have been linked to the prevalence of the pathogen such as: presence of host plant, frequent rainfall/irrigation and increased temperatures in spring and summer. In addition, a reduction of drainage of the soil due to various techniques such as soil compaction is also known to create favorable environments for the pathogen. The pathogen is dispersed as sclerotia, and these sclerotia can travel by means of wind, water or soil movement between host plants. Diseases caused by this pathogen are more severe in soils that are moderately wet and a temperature range of 15-18 degrees C.

 

Cotton Blight
Cotton Blight

Management

It is not possible to completely control Rhizoctonia, but the severity of the pathogen can be limited. Successful control of Rhizoctonia depends on characteristics of the pathogen, host crops, and environment. Controlling the environment, crop rotation, using resistant varieties, and minimizing soil compaction are effective and noninvasive ways to manage disease. Planting seedlings in warmer soil and getting plants to emerge quickly helps minimize damage. Crop rotation also helps minimize the amount of inoculum that causes Rhizoctonia Solani. Minimizing soil compaction is also another way to reduce risk of the pathogen because this helps water infiltration, drainage, and aeration for the plants.

One specific chemical option is a chemical spray PCNB which is known to be the best solution to reducing damping off of seeds on host plants. To minimize disease, we can use plant certified seed that is free of sclerotia. Seed growers should look into only purchasing sclerotia free seeds when planting their crops since sclerotia can over winter in the soil and may not show symptoms right away. Although fungicides are not the most effective way to manage this pathogen, there have been a few that have been approved by the USDA for control of the pathogen.

 

 

Control by Fungicides

One should consult their chemical representative on which group of fungicides would be most effective with their crops in regard to Rhizoctonia solani. Spray the infected cotton field by the application recommended fungicide viz.

Metalaxyl + Mancozeb @ 2.5g/liter of water OR

Thiophanate methyl @ 2.5g/liter of water OR

Copper oxychloride @ 3g/liter of water OR

Difenoconazole @ 20ml/liter of water OR

Propineb @ 125g/liter of water.

 

By: Shehzad Ahmad Kang, Department of Plant Breeding and Genetics, University of Agriculture, Faisalabad, Pakistan.

Corresponding author’s email; shehzadpbg@gmail.com

 

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