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Cercospora leaf spot disease is a fungal disease. It is caused by the pathogen Cercospora beticola Sacc.. Cercospora is the leaf disease of sugar beet with the highest impact worldwide. High moisture and warm weather are favouring the spreading of these leaf spots.
The fungi Cercospora survives in leaf debris in the soil. The sclerotia-like mycelium can survive for up to two years and forms conidia in spring, which can trigger the initial infection.
Typically, dark conidiophores with clear conidia (spores) can be seen inside the leaf spots as light gray. These are the structures the fungus uses for spreading. They are spreading by wind or water within the population. During the infection, the germ tube formed out of the fungal spores, penetrates the stomata of the leaf cells. The fungus continues to grow into the tissue. The fungal hyphae make their way between the cells. With the help of haustoria, they tap into the cells. The cell liquid supplies the fungus with nutrients. In the process, also toxins are released by the fungus, plant cells die and the leaf tissue becomes necrotic. The inner area of the Cercospora spot is enclosed by a dark red to brown ring. (Graph)
Conidia formation and infection by the fungus find the most beneficial conditions in warm and humid weather: temperatures between 25 and 30 ° C and a humidity of 100% are optimal. However, growth and spread are also possible between 6 to 35 ° C with a humidity> 91%.
Depending on climatic requirements, the disease occurs regularly in the south and south-west of Germany, preferably in the river plains (e.g. Danube, Rhine). In other regions, a stronger infection can be detected in warmer years or in specific areas whose microclimate favour the development of the fungus.
The Ramularia leaf spot is also a fungal disease. Ramularia occurs less often in Germany.
The Ramularia beticola Fautr. et Lamb causes the Ramularia leaf spots. The Ramularia leaf spots can be confused with Cercospora leaf spots. However, Ramularia spots are larger and lighter. The definite distinguishing feature is the color of the conidiophores, which are visible as dots in the leaf spot: a Ramularia spot has light conidiophores, in contrast to the dark conidiophores of the Cercospora spots. The temperature and humidity optimum for the development of Ramularia is lower than for Cercospora. Temperatures between 16 and 20 ° C and a humidity of 70% are favourable for the occurrence of Ramularia.
In Germany, the disease occurs more often in autumn, when Cercospora and Ramularia can also occur on the same leaf at the same time. However, also mixed infections of Cercospora, Ramularia, powdery mildew and rust can be observed frequently in this time.
Powdery mildew is a fungal disease. The pathogen is the fungus Erysiphe betae. The fungus is observed in all sugar beet growing regions around the world. It is of particular importance in areas with dry and warm climates.
Powdery mildew, Erysiphe betae, forms a mycelium only on the leaf surface with gray-white conidia. Hyphae penetrate the epidermal cells and attach the mycelium on the leaf with the help of haustoria. The fungus supplies itself with nutrients from the haustoria. (Diagram) The mycelium is initially perceived as a spider web- like structure on the leaf surface resulting in a powder-like covering which is looking like flour. The white gray covering can be washed off. The conidia of the fungus are spread by wind.
In contrast to Cercospora, powdery mildew does not lead to the death of cells and tissue. Loss of yield is caused by its parasitic way of life.
And then again in comparison to Cercospora, the occurrence and spread of powdery mildew is favoured by warm, but dry weather. Powdery mildew infestation can cause yield losses that are as high as those with Cercospora leaf spots. An early infestation in July must be treated according to the threshold of infection. The plants should be checked by the end of August.
Beet rust is a fungal disease. It is caused by the pathogen Uromyces betae. Beet rust is rarely seen in sugar beet in Germany. As a rule, it then appears from early autumn. Sometimes there is an early infestation in July. Its special weather requirements limit or promote its appearance and spread greatly. This can lead to noticeable "rusty years" in beet cultivation.
The fungus survives with the help of "winter spores" on leaf debris, which are in the soil from the last year. In spring, these "winter spores" - the teleutospores - germinate and create basidiospores that infect the beet plants. There is no change of host. Also spore beds (aecidia) can form on the underside of the leaves. The aecidiospores produced in it germinate and produce the clearly visible rust-red pustules on the beet leaves. Here the uredospores "summer spores" are produced. When the pustules burst, the spores emerge like red- rust dust. High humidity and temperatures between 10 and 22 ° Celsius are ideal for their development. Higher temperatures are counterproductive. In autumn the darker teleutospores "winter spores" are produced from the uredospores and the cycle continues.
Rust spots are relatively often found in beets in autumn, mostly mixed with powdery mildew or ramularia. The harmful effect is then usually negligible. The spores are spread by the wind, but rainfall also spreads the disease in the population.
The "Syndrom basse richesse" (SBR) is referred to the syndrome of low sugar. The disease is transmitted by Pentastiridius leporinus (leaf hopper) by transferring a bacterial pathogen complex into the plants. The yield losses can be very significant.
SBR was first observed in Burgundy, France, in 1990. The disease made its way through Switzerland to southern Germany. The carrier of the bacterial disease is the reed leaf cicada. If the sugar beets have been infected with the bacterial pathogen complex (a proteobacterium and a phytoplasm), the pathogens multiply and block the vascular system of the plants. The nutrient balance is disturbed. Chlorosis forms on the leaves and as the infection proceeds, the leaves are dying. The new growing leaves cost the sugar beet a lot of energy, which is gained by the stored sugar.
In contrast to the black bean aphid, the green peach aphid occurs in much smaller numbers. The green peach aphid is dangerous for beet cultivation because it transmits yellowing viruses.
The green peach aphid (Myzus persicae) can transmit dangerous yellowing viruses to sugar beet during its sucking activity. The green peach aphid is rare, but the virus potential is much higher than that of the black bean aphid. The damage threshold until row closure is 10 % infested plants. Green peach aphid is found preferentially on the heart leaves inside the leaf rosette. As it seldom forms colonies, the plant must be closely examined for infestation.
Control: After the ban on neonicotinoid seed dressing in 2019, aphid monitoring (Nordzucker, LIZ) was established, starting at the end of April. After exceeding the damage threshold, an insecticide measure is necessary.
Several hundred plant species belong to the summer host plants of the green peach aphid. Besides sugar beet, potatoes, rape, cereals and many more are also colonised. The main host in winter is the peach.
The occurrence of the green peach aphid depends on winter temperatures and spring weather. The green peach aphid usually overwinters as an egg on the winter host, but live overwintering is also possible. In spring, initially unwinged aphids reproduce on the winter host, only the following generations are winged and fly to the summer host plants. On the winter hosts of the green peach aphid, the development and the time of flight to the summer hosts (sugar beet) can be observed.
Live overwintering in the anholo cycle has a particularly high pest potential because the aphids are viruses that cause yellowing: As soon as they colonise the sugar beet in spring, they infect the plants. The first aphid generations in the holocycle, which overwinter as eggs, are initially virus-free; they become infected with the yellowing viruses on contaminated plant material.
Virus Yellows can be caused by five different viruses. The two most common viruses are Beet Mild Yellow Virus (BMYV) and Beet Necrotic Yellow Virus (BYV). The viruses are transmitted by aphids - primarily the "green peach aphids".
The green peach aphid can transmit up to five different yellowing viruses to sugar beet. The mild beet yellowing virus (BMYV) and the necrotic yellowing virus (BYV) are the most common. They multiply and spread in the plant. As a result, the vascular bundles are blocked and the nutrient supply is disturbed. The typical yellowing of the leaves occurs and the photosynthetically active leaf area is reduced. Yield losses of 30 to 50 percent can occur.
Yellowing usually occurs in nests in the summer stand, but rarely entire stands with yellow discoloured leaves can be observed. The first yellow nests appear from the end of June, depending on when the aphids attack in spring. The beets in the yellowing nests remain behind in growth. The yellow leaves stand rigid, when crushed the disease-typical "cracking" is produced. As the disease progresses, the leaves turn increasingly orange to brown from the edges. Alternaria sets in. In the case of heavy infestation, Virus Yellows can occur all over the country (France 2020).
In 2023, the first yellowing virus-tolerant varieties were approved in the EU.