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Diseases

WHERE DO PLANT DISEASES COMES FROM ...... ??

Fungal diseases like PythiumPhytophthoraFusariumVerticilliumBotrytis and Erysiphe are often topics of interest. The common names of these fungal diseases are often descriptive of the symptoms that are exhibited (damping off, wilt, grey mold, powdery mildew). Damping off describes young seedlings attacked by the fungus Pythium, they keel over at the base of the stem as if somebody has pinched them. Growers will describe the symptoms they have observed in their crops, which are caused by specific pathogenic fungi.

Grower often ask where do these disease come from. To answer this question, it is best to start at the beginning. 

What is an inoculum? 

Inoculum is the source of plant pathogen which is capable of starting an infection in plants. It can be a single spore or strands of mycelium. For example, Botrytis(grey mold) infections can start from one spore landing on an injured surface. Once Botrytis infection has started there may be billions of spores present in one inch of infected tomato stems. 



Where this inoculum comes from? 

Air, water and soil are the major sources of inoculum. Botrytis spores are present in the air as are Fusarium spores. They come from diseased plant material present in soil. Pythium is present in soil and water. If a dugout not lined with plastic material is used as a water source, a lot of soil particles are suspended in the water. This water can be a major source for Pythium

There was a time when greenhouse crops were primarily grown in soil and Fusarium wilt was a big problem. Growers would steam pasteurize the soil, but soon found that the disease severity had increased. By monitoring the air, researchers found high numbers of Fusarium spores and these spores could colonize pasteurized greenhouse soil quickly before other organisms can take hold. There has been cases of white mold (Sclerotinia) being brought into the greenhouses by honey bees visiting from the canola fields outside. Fungus gnats can carry the spores of Pythium with them. Several insects are known to carry viruses into the greenhouses. 

How do these fungi multiply? 

Most of these fungi do not have chlorophyll, the green pigment primarily in leaves, therefore they cannot carry out the process of photosynthesis. They must get food from other sources like inorganic nutrients and carbohydrates to live and multiply. Take Pythium for example. It produces motile spores which swim in water and find a root tip (especially a young root hair) and attach themselves to it. The zoospores obtain nutrients from the young root hair which is rich in nutrients for its own growth. Root exudates provide the carbohydrates as a source of energy for these zoospores. One zoospore can start the infection process. The spore germinates and produces mycelium which can infect from one cell to another cell. Water logged conditions in the root zone encourage the multiplication of this fungus. Under nutrient stress the fungus produces "hardy" spores which can survive for a long period of time.



Other fungi have their own mechanism of producing spores for disease spread. Botrytis is considered to be a weak pathogen. It enters the plant through a wound like a pruning injury or attack very young tissue like immature fruit. To start the infection process the spores land on the injured surface, with the addition of a droplet of water they germinate. Condensation is the most common source of the water droplet. Condensation occurs when the relative humidity is over 90% in the greenhouse and the temperature drops to a level where dew is formed. The dew drops accumulate on plant surfaces like leaves. The heat loss is rapid on the leaves because their large surface area acts as a water condenser. Within eight hours the Botrytis spores will start to germinate. The germ tube which comes out of the spore produces powerful enzymes which macerates cell walls allowing access to the cell saps, rich in nutrients. These nutrients are used for multiplication. 



Fusarium can come into the growing medium from water. The fungus obtains carbohydrates from root exudates and starts the infection process. Millions of spores can be produced on infected tissue. The fungus can block the water conducting system causing the plant to wilt. For example, once the top part of a tomato plant starts wilting, symptoms like blossom end rot, a calcium deficiency, can develop. Nutrient deficiency symptoms in upper parts of the tomato plant may develop because of nutrient blockage by the fungus. 



So what can we do to control these diseases?

Check for the source of the disease causing organisms. If your dugout water seems to be the source, consider chlorinating the water supply. Sand filtration has been found to be effective in reducing the populations of Fusarium spores. If recycling the water, consider heat pasteurization, ozone treatment, hydrogen peroxide and/or U.V. sterilization procedures. 



Good Greenhouse Practices (GGP). These practices include every aspect of plant growth which makes the plant healthy and strong so that it is less susceptible to diseases.

§  Good water quality.

§  Good growing medium with good water holding capacity and good air porosity.

§  Choosing a variety which has some disease resistance (if available).

§  Growing a healthy, disease and insect free seedling.

§  Adequate light is the key in the winter.

§  Using proper cultural practices to promote vegetative or generative growth.

§  Making sure that plants get what they need and in proper ratios.

§  Insect and disease scouting practices. If your greenhouse is large (an acre or more) consider hiring a part time person to monitor the crop.

§  Make correct diagnosis at the right time.

§  Improve your knowledge and skills constantly. Growers who follow GGP, travel to conventions to pick up knowledge and information then apply this knowledge to their crops.

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