Patch diseases become more severe as duration of leaf wetness increases.

Most of the fungal and bacterial turfgrass diseases that superintendents and grounds managers battle are known to be more serious under prolonged periods of leaf wetness.

Bacteria require a moist film on the leaf surface to increase their population and gain entrance to the leaf's interior through wounds and stomata (breathing pores). Fungal spores (most but not all) require this wet leaf surface to germinate and form the structures necessary to locate and enter stomata or otherwise forcibly penetrate the plant tissue. Some fungal-like organisms produce a motile spore that swims in this film of water entering plant tissue through stomata.

Once the pathogen enters the plant, infection and disease can result.

Quick Tip

In addition to a wet leaf surface, if an additional energy source is available to the pathogen, the chance for disease increases. The most common energy source available to these pathogens consists of the sugars, starches and amino acids that ooze from glands called hydathodes onto the leaf surface during the evening and night hours. This exudate (guttation fluid) makes up only 25 percent of the dew that forms with the other 75 percent being condensation from the atmosphere. When turfgrass managers reduce the concentration of the guttation fluid, disease problems are lessened.

Dollar spot was controlled for many years by a hose dragged over golf greens or a bamboo pole used to knock the dew off the grass. Some turfgrass managers use this same technique today. Fiberglass has replaced bamboo as the pole of choice and, when done early in the morning, diseases can be prevented or at least reduced in severity. This procedure knocks guttation fluid off the leaf blade, thereby reducing the energy available to the disease pathogen.

Knocking the dew off the grass also hastens the drying of the grass, reducing the amount of time the pathogen has available for infection to take place.

Bacterial diseases increase in severity in direct relationship to the length of time the leaves are wet. Infection by bacterial pathogens is more severe under shade conditions than in nonshaded areas because of an increase in humidity within the turf canopy and the increase in the length of time the leaves remain wet.

Rusts (Pucciniaspp.) and leaf spot fungi (BipolarisandDrechslerspp.) are more severe in heavily shaded grasses than in areas with full sun exposure. This is again because of the duration of leaf wetness.

Moisture on the foliage determines the production and survival of most fungal spores. As early as 1930, the incidence of brown patch, caused byRhizoctonia solani, was shown to coincide with irrigation in the afternoon. The severity of this disease is known to increase when the length of leaf wetness extends beyond nine hours.

The longer the leaf surface is wet, the greater the risk of infection and the greater the number of infections per leaf. Minimal infection by this disease pathogen occurs when the duration of leaf wetness is below six hours; severe infections occur beyond eight to 10 hours.

Irrigation in the afternoon is directly associated with an increase in infection, especially when warm day temperatures are followed by cool night temperatures. When the turf does not dry out before nightfall, normal dew formation during the night hours extends the time the grass is wet. This increase in infection also may be because of an increase in guttation fluids that feed the pathogens.

Watering in late afternoon causes an increase in water pressure within the plant tissue resulting in more guttation fluid being exuded onto the leaf surface. More guttation fluid means a higher concentration of sugars, starches and amino acids available to the pathogen.

Gray leaf spot of perennial ryegrass and tall fescue caused byPyricularia griseais known to become more severe as the duration of wetness increases. Even patch diseases have been reported to increase in severity with prolonged periods of leaf wetness. The take-all patch organism (Gaeumannomyces graminis) is particularly sensitive to moisture fluctuations. Necrotic ring spot (Leptosphaeria korrae) also increases in severity with excessive moisture and frequent irrigations.