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Disease ControlClick the image below for a larger scale view.
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Table of Contents
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1. Soilborne vs. Foliar DiseaseIt may not be obvious to everyone why plant pathologists make such a distinction between foliar diseases and root diseases, but several important differences exist. The normal situation with most soilborne diseases is that the pathogen in the soil does not move, or only moves a small distance. The host (roots) must grow to the pathogen in the soil for infection to occur. With soilborne diseases the rate of disease increase is usually low. The situation is reversed with foliar diseases. In this scenario, the pathogen (spores) moves to the host. The spores usually are disseminated in the wind. After infection, more inoculum is produced which will infect more tissue and plants. Because of these situations, most foliar diseases increase at an exponential rate. Although the potential for severe loss due to a foliar disease is large, the potential for control can be much better compared to a soilborne disease. One reason for these control possibilities is that the infection sites (foliage) are exposed and can be protected. This is not the case with soilborne diseases because the roots are covered with soil. Powdery mildew fits the scenario described above. It is a foliar disease with an exponential rate of increase. Control is achieved by lowering the rate of disease increase.
2. Introduction of Disease to United StatesSugarbeet powdery mildew was unknown in the United States until the pathogen was reported in a single field in California in 1934. The disease was not again reported in this country until it became epidemic in 1974. In April of that year, the disease became widespread in the Imperial Valley of California. By September, the malady had spread all the way to Sidney, MT and was reported in all of Holly Sugar's production areas. By the following year, the disease occurred in all sugarbeet production areas of the United States. Powdery mildew may occur first in the warmer beet growing areas. The spread may be spotty, depending on the time of infection and weather conditions. When mildew occurs in late July or early August, some crop loss may occur. Late occurring mildew (September) would not be expected to cause a measurable crop loss.
3. Fungal Morphology and InfectionPowdery mildew is a fungal disease. The causal fungus is called Erysiphe polygoni. It is related to the fungi that cause powdery mildews on grain and other crops. The fungus produces spores called conidia (see Image #1). Click image for full page view. The conidia blow in the wind and some will land on sugarbeet foliage. The conidia then germinate and start growing. The fungus grows vegetatively by producing strands called hyphae (see Image #2). The hyphae first appear in small circular areas on the leaf, growing superficially on the surface (see Image #3). Click image for full page view. As the hyphae grow, these areas will coalesce and they eventually cover the surface of the leaf. Usually, the upper surface is more affected than the lower surface. The fungus derives its nutrition by adsorption through specialized hyphae called haustoria. The hyphae will grow through the cell wall of the plant and form haustoria. The haustoria invaginate the cell membrane; nutrients defuse through the membrane and are absorbed by the haustoria. The nutrients are transported through the hyphae to all parts of the fungal thallus. Soon after infection, the fungus will begin to produce conidia. The conidia are formed on other specialized hyphae called conidiophores. The conidia form on the ends of the conidiophores in short chains. The conidia are each composed of a single cell, measure approximately 2/1000 inch long by 1/1000 inch wide, and are transparent (see Image #4). Click image for full page view. When the hyphae and conidia form, the leaf will take on a white, powdery appearance (see Image #5). As the infection progresses, the leaf tissue becomes chlorotic, then brown. These symptoms usually form first on the older leaves. Eventually, all leaves may become infected. Click image for full page view. Like most powdery mildew fungi, the sugarbeet pathogen has a relatively narrow host range. Other forms of Beta vulgaris, such as table beet and Swiss chard are susceptible. Six additional Beta spp. are susceptible, but only Beta macrocarpa might be important in the epidemiology of the disease in the United States.
4. Disease Response to Humidity and TemperatureThe conidia of powdery mildew fungi are unique in their response to humidity. Most fungal spores require 100% relative humidity or free water to germinate. Powdery mildew conidia are capable of germination at any humidity. This enables the pathogen to spread during the entire season, and not just during periods of rainy weather. Disease spread does increase at higher humidity. The fungal structures are very sensitive to low temperatures and will not overwinter in the sugarbeet production areas of the Great Plains and Rocky Mountain areas. The fungus probably overwinters in the southwestern part of the country, on winter-grown sugarbeet crops, and on weed species such as Beta macrocarpa. The beet seed fields of Oregon could also be a source on inoculum. It is thought that wind currents, during the summer months, move the conidia north. These spores infect sugarbeets, produce secondary inoculum, and the disease spreads throughout the crop. Some of these secondary conidia will be blown further north, and the cycle repeats. In this way the disease progressively moves from the southwestern part of the country throughout all of the sugarbeet production areas of the United States.
5. Crop susceptibility and Yield LossThe age of the sugarbeet crop is an important factor in susceptibility to disease. The disease is rarely seen in the field until 8 to 12 weeks after emergence. The disease occurs first on the older leaves. If allowed to go unchecked, the disease progresses and within a month will cover all the leaves in a field (see Image #6). Click image for full page view. Yield loss due to powdery mildew will occur if the disease is allowed to go unchecked. The earlier the disease occurs during the season, the greater the loss. Loss will occur due to decreased root yield as well as decreased sucrose concentrations. Gross sugar yields may be decreased as much as 38% under severe infection. In addition, powdery mildew will cause a reduction in purity. Infected plants have higher concentrations of sodium and amino-nitrogen in the roots. The decreased purity will reduce the amount of extractable sugar.
6. Disease ControlIf the onset of powdery mildew occurs a month or more before the crop is to be harvested, control is usually warranted. Because of the rapid rate at which the disease increases, control measures must be initiated at the first observation of symptoms. A delay will cause control measures to be less effective.
The sulfur fungicides offer very effective control. This material is used throughout California to control powdery mildew on sugarbeets. Not only are sulfur applications very effective for control, they are also inexpensive. sulfur dust, applied at 20-40 lb./acre will provide excellent control of the disease. Apply sulfur as soon as mildew appears, if it develops prior to mid-September. Wettable sulfur applied at 3-10 lb./acre in at least 10 gallons of water per acre also will protect the crop from serious loss. It is very important that the sulfur application be made with an adequate volume of water, a reduction will reduce its effectiveness. Often times, if the sulfur application is to be made by an aerial applicator, less water is applied. As growers, you should pressure these applicators to do the job correctly and apply the material in the correct volume, after all they do work for you. If the aerial applicator is reluctant to comply with recommendations, a ground application should be made. Sulfur controls the disease by dramatically lowering the rate of disease spread. The sulfur will decrease the production of secondary conidia, and will protect non-infected leaf surfaces from becoming infected. The non-infected tissues will remain healthy. If disease onset occurs early in the season, more than one application of sulfur may be required. Usually the second sulfur application is made 3-4 weeks after the first.
Foliar sprays of tridimefon are also used for powdery mildew control.
Before control measures are commenced, consideration must be given to the time of harvest. If the field is to be harvested within 3-4 weeks after disease onset, control is probably not necessary. If a field is being harvested on a daily quota, perhaps the disease in only a portion of field should be controlled. These are some of the variables that must be considered. Contact your Holly Sugar agriculturist for assistance. Powdery mildew is a disease that can reduce yield, but it is easy to control. By maintaining a constant vigilance on the crop, proper and timely control measures can be undertaken. The symptoms of this disease are obvious, but unless ones looks for them, they will go unnoticed.
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