Wednesday, October 24, 2007

Greenhouse - Moisture Management and Diseases

Edema or water blisters on ivy geranium.
Photo from the Utah State Pesticide Applicator Training Guide

Overwatering can be a major problem in greenhouses. The following is an article on watering for greenhouse growers.

Ever pull a seedling out of a poorly drained flat and find that it has no roots? Ever notice that diseases are more troublesome when pots are spaced too closely or when the humidity is too high? What do these two scenarios have in common? Moisture. Host plants, pathogens (or agents that cause disease), and the environment in which they occur have a close relationship that determines whether a disease will develop under a given set of circumstances. The environmental condition most likely to impact disease development in the greenhouse is moisture. Living organisms (plant hosts and pathogens alike!) consist chiefly of water, so the uptake of water is critical if these organisms are to grow.

From a host perspective, too little water in the soil (drought stress) or too much soil moisture (which leads to oxygen deprivation) places plants under stress, and plants with water stress are more susceptible to disease. Water is also important to facilitate movement of nutrients from the roots to aerial plant parts, as well as sugars, made in the leaves during photosynthesis, to the roots. Thus, any environmental condition or disease that affects the roots or vascular tissues also places undue stress on the host. Symptoms of plants with water stress include leaf wilt, yellowing (often of lower leaves), scorch, or premature drop, a decline in vigor, progressive branch dieback, and eventual death.

From a pathogen perspective, moisture extremes have a similar impact. Although too much water in the soil deprives pathogens of oxygen, too little water impedes pathogen survival and the infection process. Free moisture on leaf or root surfaces is necessary for fungal growth, germination, penetration, and dispersal of fungal spores to new hosts. Indeed, most pathogenic
fungi grow best in a damp environment.

For example, many pathogenic fungi secrete enzymes into leaf or root tissues to macerate, or soften up, host cells, releasing nutrients that are taken up by the fungi as food. Free moisture is needed to help move these enzymes and nutrients across the fungal cell wall.

Bacterial pathogens commonly enter plants through guttation fluid at the edges of leaves as water retreats within tissues during drier parts of the day. Water is also important for the fungal infection process, which is a series of steps that includes spore germination, penetration through the host epidermis, and fungal growth within the plant tissues. The pathogen can be particularly vulnerable to drying at this time.

For most aerial plant pathogens, such as Botrytis, prolonged leaf wetness facilitates the disease process. Free moisture and high relative humidity are important for infection of leaf and other above-ground tissues (such as petals, stems, or branches). This process requires a period of continuous leaf wetness — the duration of leaf wetness needed varies with the fungus. It stands to reason, therefore, that irrigation strategy would have a great impact on the development of leaf diseases.

For soil pathogens, water in soil pores is also needed for the motile spores (called zoospores) produced by the water molds (for example, species of Pythium and Phytophthora) to swim toward healthy roots. Finally, moisture, in the form of rain, splashing, and running water, is important for the dispersal of spores of all kinds of fungi, wherever they may attack, to new hosts. Only a few kinds of fungal diseases (powdery mildew and Botrytis blight) disperse easily without help from overhead irrigation.

Moisture management in the greenhouse environment

Moisture management is important from both a plant health and a pathogen point of view. Too much or too little moisture during production can have equally devastating results. Some ornamentals are prone to edema, a physiological disease that is generated by the water relationships of leaf cells. Corky, blister-like outgrowths on the lower surface of leaves occur on edema-prone plants during cloudy periods with abundant moisture. Certain nutritional and light factors may predispose plants to edema. When more water is supplied to leaf tissues than is lost during transpiration, the cells that line leaf stomates (where gas exchange occurs) become too full and burst. Some of the crops especially prone to edema are begonia, fern,
ivy geraniums, and pansy.

As a rule, even for crops not prone to edema, you should avoid prolonged periods of leaf wetness to curb the success of aerial plant pathogens. If using overhead irrigation, design and time watering events to minimize the length of time foliage stays wet. Consider using drip irrigation or aimed microemitters which do not place moisture on foliar surfaces. Water thoroughly, but less often, to keep the surface of the mix dry between waterings to reduce root and crown rots caused by Rhizoctonia and Phytophthora.

Reprinted from "Moisture and Those Troublesome Greenhouse Diseases!" by Ann B. Gould, Ph.D., Specialist in Plant Pathology, and Margery Daughtrey, Senior Extension Associate, Cornell University in the November 4, 2004 issue of the Rutgers Plant and Pest Advisory, Landscape, Nursery, and Turf edition.

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