Disease management requires growers to look at a number of factors - those that influence disease spread, increase inoculum and lead to infection.

Agronomy

Powdery Mildew

Description

Powdery 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. Powdery mildew is a foliar disease with an exponential rate of increase.

Symptoms

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The first signs of powdery mildew are small, white powdery spots that appear usually on the under surface of older leaves when sugarbeet plants are 2 to 6 months old. Under suitable conditions, the fungus spreads rapidly over the entire surface of the leaf, and eventually to all leaves on affected plants. Older leaves may yellow and eventually become necrotic and die.

Development

Powdery mildew is an annual problem on sugarbeet in California. The fungus overwinters on sugarbeet and other Beta species such as swiss chard, table beet and wild Beta species that grow throughout the winter. Ideal conditions for disease development are warm, dry weather; optimum temperatures for growth of the fungus are between 60° and 86°F (15° and 30°C). Very high daily temperatures of 100°F (38°C) or higher tend to arrest disease development.

Following initial infection, the fungus grows over the surface of the leaf and produces asexual spores (conidia), which give the leaf a powdery appearance. The conidia are airborne and can be carried considerable distances to start new infections. If the disease is not controlled, 20 to 35% loss in sugar yield can occur.

Spread

Sugarbeet 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.

Control

If 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 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.

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.

Holly Hybrids

Agronomy

Powdery Mildew