Cercospora Leaf Spot
This disease is caused by the plant pathogenic fungus Cercospora beticola. It occurs wherever table beets, Swiss Chard, sugar beet, and spinach are grown and is one of the most important diseases affecting the Chenopodium group. It can result in significant losses, particularly in late summer when conditions are favorable (high temperatures, high humidity, long leaf wetness periods at night). Leafy greens become unmarketable, and beet roots fail to grow to full size when disease is severe.
Symptoms occur as numerous, initially small circular leaf spots. Spots have a pale brown to off-white center with a red margin. Lesions expand in size, coalesce, turn gray as the fungus sporulates, and can result in extensive loss of foliage. Leaves at the center of the plant are often less severely affected. The pathogen produces sclerotia or stromata which can be seen with a hand lens as small, black dots in the center of lesions. Lesions may also occur on petioles, flower bracts, seed pods, and seeds. Leaf symptoms are similar to those caused by Beet Phoma (Phoma betae), except that the phoma will have more obvious tiny fruiting bodies in the lesions and can also affect the roots. The pathogen C. beticola survives between crop cycles in residues from infected crops (as sclerotia), in weed hosts, and on seed. It can survive in the soil for up to two years. High levels of disease can result from just a few infected plants since each lesion produces numerous conidia. Several cycles of infection and conidium production may occur with favorable environmental conditions. Spores can penetrate the leaf directly through open stomates. The pathogen is favored by high relative humidity and temperatures between 24-30˚C and is spread by rain splash, wind, irrigation water, insects, workers, and equipment. Leaf wetness during the night, even with dry conditions during the day, encourages disease. Successive plantings made close together can allow disease to move from one planting into the next.
Model for prediction Cercospora Leaf Spots
Because of the strict environmental conditions needed for an infection the disease is predictable within a time period, when outbreaks are mostly favorable. The prediction system is an estimate of the potential for disease development based on the relative humidity and temperature measured within fields. This system was developed in the late 1980s by UNL scientists, Albert Weiss and Eric Kerr, and still is being used today at the University of Nebraska Panhandle Research and Extension Center. Over the last 15 years the forecasting system has utilized up to 14 sites per season located in Nebraska, Colorado, Wyoming, and Montana. Results are then collated and disseminated to more than 40 sources, including consultants, researchers and media (Web, print, television, and radio). This system assumes that a susceptible host and sufficient inoculum are present. Based on hours of leaf wetness or high relative humidity (> 90 percent) and temperature during this period, a daily infection value (DIV) is determined (Table I).
If the two-day sum of the DIVs is seven or greater, there is a strong potential for infection and further disease development. If the sum is less than six, there is little likelihood of infection. The following example will illustrate how to use the information in Table I. Assume that on Day 1 there were 13 hours of leaf wetness and the mean temperature during this period was 63 o F. On Day 2, there were 15 hours of leaf wetness with a mean temperature of 65 o F. The DIV for Day 1 was three while on Day 2 it was four. The sum of these two days was seven, resulting in conditions that would favor infection. If no symptoms were observed on leaves, then the DIV sum indicates that careful scouting is advised, and if symptoms were present, then a fungicide application would be warranted. If on Day 3, there were 12 hours of leaf wetness and the temperature during this period was 62 o F, then the DIV sum for Days and 2 and 3 is 4+0=4, and no action would be necessary.