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On 16 Jul 2005 11:03:40 -0700, wrote:
respected university agricultural services http://www.coopext.colostate.edu/TRA...S/leafwet.html Watering Turfgrass and Disease Potential: Leaf Wetness Curtis E. Swift, Ph.D. Colorado State University Cooperative Extension Tri River Area -------------------------------------------------------------------------------- Introduction: Most fungal foliar disease pathogens require specific leaf surface conditions for their spores to germinate; favorable temperature and a film of water on the plant surface (or a high relative humidity) are required. These conditions must last sufficiently long enough for the pathogen to penetrate the plant, otherwise the germinated spore dries out and dies (Agrios, 1988. p. 45). The presence of dew on the leaf surface and extending this natural period of leaf wetness results in increased fungal growth (Smiley, et al. 1993, p. 78) and the presence of foliar diseases (Leslie, 1994, p. 389). Dew on turf consists of condensation from the atmosphere (~75%) as well as guttation/exudation fluids from the leaves (~25%) (Williams et al., 1998). The presence of sugars, starches and amino acids in the guttation fluids exuded from the plant provides an energy source for the invasion of the plant by the pathogen. Bacterial diseases increase in severity in direct relationship to the length of time the leaves are wet (Sirjusingh, et al. 1996; Zehr et al. 1996). Infection of turf by bacterial pathogens also increases under shade conditions due to the increase in humidity within the turf canopy and the increase in the length of time the leaves remain wet (Giesler et al., 2000). Rusts (Puccinia spp.), powdery mildew (Erysiphe graminis DC.) and leaf spot fungi (Bipolaris and Drechsler spp.) also are more severe in heavily shaded grasses than in areas with full sun exposure (Beard, 1965; Smiley et al., 1993). Beard estimated that 20 to 25% of all turfgrass is shaded to some degree by trees, shrubs, or buildings (Beard, 1973). With fungal diseases, moisture on the foliage determines the production of spores and their survival. Gross et al., report the severity of brown patch (Rhizoctonia solani) increases as the length of leaf wetness increases above 9 hours. The longer the leaf surface is wet, the greater the risk of infection and the greater the number of infections per leaf. Fidanza et al., report minimal infection of this disease occurs when the duration of leaf wetness is below 6 hours with severe infections occuring when the length of leaf wetness increases to 8 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 (Dickson. 1930). Once the optimum temperature is reached gray leaf spot of perennial ryegrass and tall fescue (Festuca arundinacea Shrebl) caused by Pyricularia grisea (Cooke) Sacc. increases in severity with an increase in the length of leaf wetness (Uddin et al. 1997; Moss & Trevathan, 1987; Williams et al, 2001). Even patch diseases have been reported to be more severe with prolonged periods of leaf wetness (Fidanza, et al. 1996; Giesler, et al., 1996) The take-all patch organism (Gaeumannomyces graminis) is particularly sensitive to moisture fluctuations and high moisture levels in the surface layer of soil must be maintained for the infection of the grass plant to occur (Clarke, et al.). Necrotic ring spot has also been shown to increase in severity with excessive moisture and frequent irrigations (Chastagner, 1985; Smiley, 1980). The powdery mildew fungi are an exception as they require high humidity without the film of moisture on the leaf surface for germination of spores and infection to occur. While longer periods of leaf wetness are expected to occur in irrigated vs. non-irrigated turf regardless of mowing height (Williams et al., 2001), mowing height has an impact on the humidity within the turf canopy. Higher cutting heights result in increased levels of humidity that last for a longer period of time. This can result in a more suitable environment for infection by pathogens (Giesler et al., 2000). In addition, the amount of water and the timing of its application can prevent or contribute to disease development (Leslie, 1994, p. 388). Turf is not the only plant type where the length of leaf wetness influences the infection rate. Gray leaf spot of maize caused by Cercospora zeae-maydis, a major foliar disease is known to increase in severity as leaf wetness increases (Bhatia and Munkvold, 2002). Even apple scab due to Venturia inaequalis, (Harman et al., 1999), and the infection of flax due to Alternaria linicola (Vloutogloue et al., 1999) increase with the length of leaf wetness. Specifics: Ascochyta leaf blight control includes irrigating grass early in the morning hours when dew is already present (Smiley, et al. 1993, p. 11). This disease compendium (p. 14) also mentions that dollar spot occurs when dew is present by the growth of the fungal mycelium. This mycelium requires a period of leaf wetness for the cobwebby structure to develop. Watering early in the morning as previously mentioned would dilute the nutritional benefits of the dew thereby reducing dollar spot problems. Control for the "Helminthosporium-type" diseases (melting-out and leaf spot) include avoiding frequent short irrigations, especially in the evening (Smiley, et al., 1993. p. 40). Extended periods of leaf wetness are reported to be required for Curvularia, one of the "Helminthosporiums" (Brown, et al. 1972) and the severity of both the leaf blighting and crown rot phases of `Dreschlera' caused diseases are favored by extended periods of leaf wetness (Couch, 1995, p. 106). Recommendations: Practices should be followed that keep the leaf wetness less than twelve (12) hours (Couch, 1995, p. 252). Twelve (12) or more hours of moist foliage can trigger a major disease outbreak. The shorter the time the grass is wet, the less the disease problem (Leslie, 1994, p. 389). The infection of a plant by a fungal pathogen requires spore germination and development before tissue penetration can occur. The requirement of leaf wetness for these processes to occur in part explains the reason why leaf-spot is more serious in lawns on the north side of a building or in low areas where the turf remains moist for extended periods of time (does not dry out). The spores of some fungal organisms germinate producing a motile spore that must swim in a film of water before infection can occur (Agrios, 1988. p. 44). -------------------------------------------------------------------------------- |
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"Recommendations:
Practices should be followed that keep the leaf wetness less than twelve (12) hours (Couch, 1995, p. 252). Twelve (12) or more hours of moist foliage can trigger a major disease outbreak. The shorter the time the grass is wet, the less the disease problem (Leslie, 1994, p. 389). " Thanks for adding another post to the list that agree with my position. Here's my original post that you had such a vitriolic disagreement with and said I didn't know what I was talking about: "I agree. Lots of people say not to water at night, as wet grass promtoes disease and fungus. However, I think this is mostly nonsense. AFter all, it rains at night doesn't it? I think you could safely water at night, as long as it's like every 4-7 days and giving it about an inch. I think the night watering problems come from people who are watering it every night, and with only enough water to get it wet. That is wrong, wheneve you do it. I do mine exactly as Warren suggest, having it end around dawn or shortly after. That minimizes evaporation and the time the lawn is wet. " If you do what I suggested, the lawn will be watered at night, ending at 6 or 7AM, and be wet for less than 12 hours, exactly what this reference recommends. In fact, your reference says watering in the AFTERNOON increases the problem with fungus! So what the hell are you complaining about? Oh, I remember, that NJ is not the center of the universe! |
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It's best to water before 7:00 a.m or after 7:00 p.m. Make sure you water
deeply. That is more important than the time of day you water. "Joey Goldstein" wrote in message ... -- Joey Goldstein http://www.joeygoldstein.com joegold AT sympatico DOT ca |
So, you can't over water in the daytime? Overwatering and when you
water are two very different things. Of course overwatering is possible. Again, in the context of overall "simple advice" to inexperienced homeowners it's not unreasonable to make the blanket statement of avoiding night watering. That's entirely independent of overwatering or other ways of doing it wrong. Your all worked up about it for some reason. Let it go. |
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