Alternaria solani

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Alternaria solani
Infestation on tomato leaves

Infestation on tomato leaves

Systematics
Class : Dothideomycetes
Subclass : Pleosporomycetidae
Order : Pleosporales
Family : Pleosporaceae
Genre : Alternaria
Type : Alternaria solani
Scientific name
Alternaria solani
Sorauer

Alternaria solani is a fungus in the genus Alternaria . Asa causative agent of dry spot disease, the species isan important pathogen in potatoes . But it also affects other species ofthe nightshade family such as tomatoes .

A. solani often occurs in potatoes together with the closely related species Alternaria alternata , which causes the symptomatically very similar spray stain disease. In the literature, both pathogens are usually treated together under the generic term "Alternaria disease in potatoes".

biology

A. solani reproduces, like all species of the genus Alternaria, exclusively asexually. The mycelium forms septate conidia, on which the golden yellow conidia arise, individually or in small groups. The conidia, which are viable for about eight weeks, have a beak that is at least as long as the spore itself. Their shape is slightly curved and club-shaped. In each conidia there are 9–11 mostly exclusively transverse septa.

Development cycle

The fungus survives as a saprophyte on dead plant material such as harvest residues in the soil. In the course of the summer, spores form, which are locally transferred from the ground to the lower leaves of the plants via rainwater splashes. The first infestation sites appear there in early summer (end of May to end of June). If the weather conditions are favorable (usually from mid-August), spores form there, which spread over greater distances and trigger new infections.

A. solani does not spread systemically in plants. Each infected area is caused by its own infection.

Expectations

The Alternaria mushroom has comparatively high temperature requirements (20-25 ° C), which is why the rapid development in the population often only begins in midsummer (late August to early September). Dryness is also beneficial for spore formation. However, the spores need moisture to germinate. The optimal conditions are therefore longer, warm dry periods with short periods of moisture (precipitation, sprinkling or nocturnal dew formation).

In contrast to other plant diseases such as late blight, which infect plants regardless of their general condition, A. solani is not a very aggressive pathogen, but rather a weak parasite. A significant infestation usually only sets in in phases of stress, such as an infestation with other diseases, water stress, nutrient deficiency or the beginning of maturation.

Host plant spectrum

Alternaria solani is a pathogen that affects numerous species of the nightshade family (Solanaceae). It is particularly important in the cultivation of potatoes (dry spot disease) and tomatoes. In addition, some cruciferous vegetables, such as lettuce, can also become infected.

Pathogen causing dry spots on potatoes

meaning

Alternaria solani has been known to cause dry spots in Central Europe for decades in potato cultivation. However, the disease did not appear economically in Germany for a long time. Significant damage has only been observed since the beginning of the 21st century. In North Rhine-Westphalia , the potato disease warning service first considered the alternative infestation in 2002. Since then, the pathogen has regularly reached a level of infestation relevant to phytosanitary issues, especially in 2006 and 2011, whereby the damage potential of the most important potato disease, late blight , was far from being reached.

In some federal states, the chambers of agriculture assume that the aggressiveness of the disease could increase further in the future. In others, the fungus is classified as a weak parasite, which should not be overrated if the culture is well managed. In any case, the disease "Alternaria in potatoes" is a current topic in potato research.

Since the Alternaria problem in potato cultivation has increased, differences between the two Alternaria species involved have also increased. For a long time A. solani and A. alternata were considered to be co-occurring fungi, which together cause the disease. It is now known that both fungi also occur individually, have different infestation times and differ slightly in their damage pattern. The disease "Alternaria on potatoes" is therefore now often divided into dry spots ( A. solani ) and spray spots ( A. alternata ). In 2010, monitoring projects and laboratory analyzes were introduced in NRW to differentiate.

Symptoms

A typical symptom of the disease are the chocolate-brown necroses on the leaves (dry spots). These tiny spots quickly become larger up to an extent of about 2 cm. If the infestation is severe, the spots grow together and can destroy the entire leaf mass. Individual spots can also form on the stems.

The spots tend to be round, but are usually bordered by leaf veins, so that they take on an angular basic shape. They are always clearly delineated from healthy tissue. In most spots, concentric ring structures are also visible, which can already be seen with the naked eye in larger leaf spots. The dead tissue is brittle, so it can easily tear or break out in parts.

In particular, the first infestation sites in early summer before the rows are closed are often overlooked, as the increase in leaf mass in the potato is particularly high and the infection rate is rather low during this period.

The disease on the tubers is called hard rot. Places on the tubers sink in, become hard, rotten and turn brown. They are sharply demarcated from the tissue that is still healthy. A systemic infestation of the tubers via the ducts, as occurs in late blight, does not occur in A. solani . Alternaria spores also cannot actively break through the shell of healthy tubers. The pathogen is therefore dependent on injuries in order to attack the tubers. Typically, this infection only occurs during harvest, which is why the symptoms of disease on the tubers only appear during storage.

The susceptibility of potatoes to A. solani is generally very dependent on the variety. Since the disease often does not set in until late summer, its damaging potential is particularly high in late varieties.

Differentiation from spray stain disease

The spray stain disease A. alternata is often closely associated with A. solani . Often both diseases occur at the same time. The symptoms are also very similar, which is why a laboratory analysis is necessary for an accurate differentiation. In the field, however, the two species can be separated based on a few characteristics: The spray stain generally occurs earlier. The peak of the infestation is often already in June, i.e. up to eight weeks before the crucial period of A. solani infection . In addition, the necroses in A. alternata are much smaller. These are not isolated, larger spots, but extremely numerous point necrosis a few millimeters in size (“spray infestation”).

Combat

The dry spot disease can be regulated very effectively indirectly through the choice of tolerant varieties, adherence to a crop rotation and good cultivation management with a balanced supply of nutrients and water.

In addition, numerous fungicides that are routinely used against late blight are also effective against Alternaria species. In conventional cultivation, the disease is usually fought with two to three targeted treatments (observe warning notices) during the potato growth phase. However, some Alternaria strains with fungicide resistance have already appeared in Germany.

literature

  • Horst Börner: Plant diseases and plant protection. (= UTB 518). 8., newly revised. and act. Edition. Verlag Eugen Ulmer, Stuttgart 2009, ISBN 978-3-540-49067-8 .

Web links

Commons : Alternaria solani  - collection of images, videos and audio files

Individual evidence

  1. Dürrfleckenkrankheit , Pflanzenenkrankheiten.ch, accessed on February 13, 2016.
  2. ^ Information from the North Rhine-Westphalia Chamber of Agriculture , accessed on February 12, 2016.