Black-leggedness
The blackleg is a plant disease .
Grain
The blackleg in grain is caused by the fungus Gaeumannomyces graminis . The wintering of wheat, barley and rye is particularly affected. The fungus causes the eponymous and typical black coloration of the grain roots. After the young grain roots are infected, they rot and, in particularly severe cases, prevent the plant from being supplied with sufficient water and nutrients. The survival of the fungus in root and stubble remains can be counteracted well by good biological activity in the soil and a crop rotation on a non-host plant, as its survival time is no longer than a year.
potato
The blackleg is a disease that potato plants attacks. The pathogen, the Erwinia carotovora bacterium (the scientifically correct name is Pectobacterium carotovorum ), causes both black-legged and “wet rot”. Black-leggedness leads to yield reductions if a stand has around 15% deficits. The tuber rot is one of the dreaded storage diseases. Significant problems can also arise with early potatoes, especially when loose-skinned goods are harvested at high temperatures and are exposed to strong temperature differences throughout the chain up to the consumer (condensation water formation). Losses in stored goods can occur more frequently in years with a very damp second half of the vegetation. Potatoes on heavy soils are more severely affected than on light locations, and particularly high losses occur in waterlogging. Experience has shown that problems with individual varieties occur much more frequently (e.g. “ Nicola ”, “ Satina ” and “ Adretta ”) than with other varieties (e.g. “ Solara ”).
Symptoms
The Erwinia carotovora infestation is a frequent cause of defects in the stand. Plants or individual shoots in the stand turn yellow, take care of and eventually die. They can be easily pulled out of the earth. The bacteria primarily destroy the cells that make up the supporting tissue of the stem. At the base of the stem there is a typical black coloration with severe tissue destruction of the so-called vascular bundles and a pungent odor. The disease prevents the upward water supply and the removal of the assimilates formed in the leaves into the tubers. The tuber rot transforms the tubers into a soft rotten, pulpy mass that is only held together by the thin shell and immediately bursts when light pressure is applied. However, the typical pungent odor is only triggered by secondary infections with certain other bacteria.
Biology of the pathogen
Understanding the biology of Erwinia is made more difficult by the fact that different subspecies of the bacterium occur, each of which can colonize different tissues of the potato plant and also differ in their mode of transmission. In addition to the potato, Erwinia carotovora also attacks a number of other plants such as carrots , cabbage, tomatoes , peppers , pumpkins , onions , beans , peas and beets .
The most important source of spread for both subspecies is a latent (= invisible) infestation of the planting material, all tubers are latently infected. The shell and lenticels (respiratory openings) of the tubers are particularly heavily colonized. The different parts of the planting material differ only in the extent of the colonization. The question of transmission via the ground has not yet been conclusively clarified. The soil appears to be a major source of infection only with a very close succession of hosts (e.g. carrots after potatoes or vice versa).
If the bacteria from the mother tuber get directly onto the shoot and grow up the shoot, they cause the typical symptoms of black-leggedness there. The development of imperfections or black-leggedness is favored above all by moist conditions after planting and during youth development. Stocks that were planted early and whose emergence was delayed by cool, wet weather are therefore most affected by black-leggedness. Precipitation and low temperatures lead to increased infestation in spring.
Germs adhering to the tuber shell cannot independently find their way into the daughter tuber, therefore an infection of the tuber tissue is only possible directly via the mother plant, or with the soil water via wounds and via the lenticels. A high soil water content creates favorable opportunities for spreading and promotes colonization of the lenticels, as these are open when there is high humidity. Therefore, heavy sites are more affected by wet tuber rot than light ones. Injuries during harvest and sorting work create entry gates for the bacterium, contact of the tubers with contaminated material (rotting tubers, dust, machine parts) during harvest, processing and storage increases the degree of contamination of a tuber or a lot. Outbreaks of wet rot, which are triggered by damage as a result of mechanical injuries during harvest and storage, are limited to the initial phase of the storage period.
Whether rot actually breaks out after the tubers have been successfully colonized depends on the number of bacteria that have penetrated , the resistance reactions within the tubers and the conditions in the warehouse. Because at Erwinia the environmental conditions (humidity and temperature) play a decisive role. It develops at temperatures above 10 ° C, the temperature optimum is 25–30 ° C. High humidity and low oxygen concentrations in the room air favor an outbreak, as the lack of oxygen almost completely inhibits the resistance reactions of the potato. A certain bacterial density is required for the formation of rot (an average of 10,000 germs per gram). This is explained by the fact that if bacteria enter the tuberous tissue via damage or via opened and uncorked lenticels, they have to produce a sufficient amount of enzymes within the short period of time before a new cork layer forms, which drives the destruction of the tissue to win the race. A certain amount of bacteria is necessary for the adequate production of enzymes, which together overcome the resistance reactions of the tubers. Under optimal conditions for the bacteria (no oxygen, moist), less than 100 bacteria per gram are sufficient to cause an outbreak of disease, while under aerobic conditions at the usual temperatures in the warehouse, at least 1,000,000 to 10,000,000 bacterial cells per gram are necessary. A high nitrogen supply also increases the risk of wet rot outbreaks, as this later corks the lenticels and provides the bacteria with an entry point for a longer period of time.
Control and preventive measures
The fight against this bacterial disease begins with the use of as little infected seedlings as possible. Precipitation and watering before the plants emerge greatly increase the risk of infestation. Harvesting should only take place after the peel strength has been achieved in dry soil. The soil conditions for harvesting and the storage conditions in the first weeks after storage are of paramount importance for the occurrence of storage rot. In order to avoid injuries, the lifting work should be carried out at ground temperatures of over 10 ° C, but the temperature in the ground should not exceed 20 ° C.
A wound healing phase of ten to fourteen days as well as careful drying of the tubers after harvest can help reduce the pressure of infection in Erwinia. An autumn sorting of the tubers - if it cannot be avoided - should be carried out with care. It represents an additional significant burden.
Good ventilation during the entire storage period ensures a supply of oxygen and prevents the formation of condensation and liquid films on the surface of the tuber as well as anaerobic conditions in the hive.
Loose-peeled new potato batches, which are very susceptible to injury, also pose a particular problem in organic farming. The loose peel means that such areas sweat more than hard-skinned potatoes in autumn when the temperatures are right. To avoid wet rot, new potatoes should be taken out of the ground as cool as possible so that the bacteria in the tubers cannot develop as quickly. This means that the lifting work should usually be completed early in the morning before 10 a.m., because in midsummer the soil temperatures rise rapidly to 30 ° C and above. Furthermore, during the interim storage in the warehouse and in the shop, a film of water should be prevented from forming on the tubers. Washed potatoes are also particularly at risk. These areas should be dried as quickly as possible after washing. Measuring devices can be purchased in stores that measure the moisture content in the stack by determining the electrical resistance.
Individual evidence
- ↑ Diepenbrock, Ellmer, Leon: Agriculture, plant cultivation and plant breeding. UTB basic knowledge Bachelor, 2nd corr. Edition 2009 Stuttgart, p. 42f.
- ↑ Gerda Bauch, Bavarian State Agency for Agriculture: Black-leggedness a problem in potato growing (PDF), accessed May 5, 2017.
- ↑ Pictures on the black-legged potato at Bing