Cercospora Leaf Blight
Cercospora is a widely known and studied genus of Ascomycete fungi. There are dozens of species identified within Cercospora, and potentially hundreds of species of Cercospora that have yet to be identified across the world and its plants. Pathologists continue studying this fungi closely.
As a disease in cultivated crops, Cercospora occurs primarily in the form of leaf spots. It can also infect the seeds and shoots of certain plants in the same manner in which it forms as a leaf spotting disease. In many crops, the disease is merely an annoyance; but in others, it can be disastrous when field, weather, and crop conditions combine to support its spread.
Cercospora in Soybeans
In soybeans, Cercospora kukuchii, is the culprit. This later season disease shows up after pollination in soybeans and during pod fill. Its signature symptomology is a give away by virtue of the purpling and bronzing of leaves located in the uppermost portion of the canopy. The leaves become almost leathery as the season progresses and the coloration deepens within the leaf. The seeds of the soybean plant can also be infected, and when this occurs, the common name of the disease is purple seed stain. The disease is most commonly found in Southern locations and in the North Central states.
Scouting and Identification When scouting in late season, Cercospora can be mistaken for other conditions and diseases such as sunburn or sudden death syndrome. Other fungi like Diaporthe and Phomopsis which cause pod and stem blight can also be confused with Cercospora, as can early onset of senescence.
However, if you closely examine individual leaves, you’ll see the unique symptoms described above. At maturity, check the seed for purple discoloration, which is the seed phase of the disease.
Significant yield loss is rare across areas where Cercospora leaf blight is found in Soybeans; however, where seed infection is significant, the value of the crop can be reduced and dockage if harvested for grain; or outright denial of certification if grown for seed. When severe, the impact from kukuchii or sojina on germination and/or seedling vigor can be significant.
Management options for Cercospora in Soybeans integrate cultural practices and host resistance. Rotation of crops to minimize buildup of the disease, clean tillage, destruction of residue, using disease free seed, locating fields to maintain good air flow, and avoiding poorly drained low lying fields all contribute to lessening the disease. Later-maturing varieties tend not to be as susceptible to Cercospora, and some varieties claim fairly good resistance in their genetics.
Chemical control of infected seed with seed treatment fungicides can be effective. Foliar fungicides which are applied during early pod stages R3-R5 may offer some control, but the decision to use them should be based mostly on fields which have a history of the disease and were planted without recommended residue controls, combined with favorable overwintering conditions. When these situations exist with current weather conditions favorable to the disease, a foliar treatment may be effective. Applying foliar fungicide outside of this recommended window is not likely to provide an economic return.
Cercospora in Curcurbits
Cercospora citrullina infects all the cucurbits, but is especially common on cucumber, cantaloupe, and watermelon. Like most Cercospora, it is usually noted infecting the leaves, but in certain seasons and weather conditions it can infect the petioles and stems. It is not known to infect the fruit of these crops.
In melons, the spots express themselves as small spots of white or grey with black margins. In other cucurbits, they develop into somewhat circular spots with tan to light brown centers, which dry out and become transparent through the leaf surface when held up to the sky. When leaf spots are numerous, they’ll coalesce and eventually turn the leaf an entire yellow. The good news is that serious economic losses in these aggressive growing vegetables is rare, and they overcome photosynthetic inhibition. Occasionally, infected plants will suffer some fruit size and quality loss, but typically below economic thresholds.
Conidia (asexual spores) of Cercospora citrullina are transported by moist winds. In order for plants to become infected, the soil and plant environment must be very humid with free moisture and warm temperatures of 26–32°C (80–90°F).
As with most of the significant species of Cercospora, Cercospora citrullina survives on crop debris, volunteers and cucurbit weeds. The best control is prevention through clean tillage, rotation, and prudent use of fungicides if warranted by scouting and field conditions.
Cercospora in Sugarbeet, Chard, Spinach
Of all the major crops affected by Cercospora, sugarbeet and similar crops are possibly the most susceptible for economic damage in certain areas of the US including the North Central (especially North Dakota and Minnesota) and Northern states.
Caused by the fungus Cercospora beticola , extended periods of hot, humid conditions are ideal for this disease. Daytime temps between 78-90OF and night time temps above 60 OF, with free moisture whether by rain or even dew for 8 continuous hours within the crop canopy provide optimum conditions for the disease to spread. If these environmental conditions do not exist, the disease will not likely establish itself. Generally speaking, if leaves are not continuously damp for at least 11 hours, new spore infection is unlikely. Foliar symptoms includecircular spots of about 1/8 inch in diameter with centers of an ashy grey color and borders of reddish-purple to dark brown margins. Each time the fungus goes through its cycle, inoculum can build up substantially. Some areas will experience as many as 5 growth and sporulation cycles in a given season.
The potential for significant loss in tonnage and extractable sucrose in sugarbeet crops is great when conditions are right for Cercospora. Therefore, timely application of fungicide is generally recommended at the first sign of symptoms, especially if weather patterns are favorable for its spread. Delaying application greatly impacts efficacy.
When old growth is damaged significantly or killed off, it’s too late and yield and/or quality losses can be expected. Fungicide controls after that point are likely futile.
Most research using fungicides has shown that sequential treatments starting at first symptoms and continued on a minimum of 14 day intervals is most effective. Consult with your Certified Crop Advisor or Crop Specialist to determine scouting protocols and timing of application. Scouting protocol combines the Daily Infection Value (DIV) and duration to come up with optimum timing. When the DIV exceeds 7 for two consecutive days and favorable environmental conditions exist, treatment is warranted. Of primary importance is the first application. If that window is missed, it is difficult to manage this disease at later points in the season. Closely follow material recommendations, tank mixes, volume of water, nozzle selection, and other protocols when treating for Cercospora in sugarbeet.
Across all the crops affected by Cercospora, cultural practices are key to success. Residue management, crop rotation, maintaining good air flow, proper scouting of susceptible crops, planting tolerant day length and resistant varieties; and if necessary, timely control practices come together to offer the best management practices against this disease and all its various species.
Read more about it:
- The Soybean Research and Information Initiative - Diseases - Cercospora Leaf Blight
- University of Minnesota Extension - Soybean - Cercospora
- Iowa State University Extension - Soybean Diseases - Cercospora Leaf Spot and Purple Seed Stain
- Siminus - US - Disease Guides - Curcurbit - Cercospora
- North Dakota State University - Crop and Pest Report - Controlling Cercospora Leaf Spot with Fungicides (08/06/15)