Pythium irregular
| Pythium irregular | ||||||||||||
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| Pythium irregular | ||||||||||||
| Buisman , (1927) |
Pythium irregulare is a soil-borne phyto-pathogen from the Pythiaceae familywithin the group of egg fungi . Egg fungi, also known as "downy mildew", are fungal protists . They are similar to fungi in terms of life cycles, but have a diploid dormancy, have coenocytic ( septicidal ) hyphae , a larger genome , cellulose instead of chitin in their cell walls, and form zoospores (asexual mobile spores) and oospores (sexual dormancy stages).
Hosts and Symptoms
Pythium irregulare is a member of the egg fungus that causes wilting as well as root rot before and after the seedling emerges from the seed. Before the seedling emerges, P. infects irregular seeds, causing them to rot and turn brown, preventing successful growth. Infection of the seedling after its emergence takes place immediately after germination. Usually this is accompanied by an infection via the root and the stem axis, which looks like soaking or necrosis . Depending on the severity of the infection, the plants can collapse completely or become seriously clumped. In older and more developed plants, P. irregulare causes root rot. This is initially associated with necrotic lesions , which lead to chlorosis , decreased yield, sparse growth and stunted growth, which is due to insufficient supply of water and nutrients from the roots. In addition, P. irregulare infections often coexist with those of other Pythium species. All symptoms caused by P. irregulare can also have been caused by other pathogens, so that a diagnosis does not necessarily lead to P. irregulare .
To unequivocally identify Pythium irregular , it is necessary to isolate the organism and examine it microscopically. First, it is important to characterize the microbe as an egg fungus by looking for the group-specific features such as coenocytic hyphae, zoospores, and oospores. Subsequently, membership of the genus Pythium can be determined by examining the symptoms of the disease, the host spectrum and the presence of vesicles that contain zoospores and are connected to a sporangium . In contrast to this, most other egg fungi do not have vesicles, but the zoospores are formed directly in the sporangia. Finally, a dichotomous key is helpful for species identification. Some of the key features of P. irregulare are irregularly shaped oogonia , cylindrical protuberances, solitary sporangia, non-filamentous sporangia, and oogonia less than 30 μm in size. There are also many molecular biological tests that provide species identification with the help of specific DNA markers . It is also important to emphasize that many diagnosticians do not determine pathogens to the species level because it can be difficult to see all of the diagnostically important microscopic structures; the recommended management measures, however, can be applied to all Pythium species.
Pythium irregulare has a very broad host range; Many agriculturally and horticulturally important crops can be found among the hosts. The pathogen is found on all continents except Antarctica . P. irregulare infects more than 200 species, including cereals , pulses , fruits , vegetables and ornamentals . The pathogen differs from other Pythium species in that it prefers cooler environments. Moist conditions are also required for the disease to break out, as this helps the zoospores to move. The species is commonly found both in greenhouse cultures and in the open field.
Life cycle
Pythium irregulare , like most egg fungus species, has a life cycle with sexual and asexual stages. During winter, oospores, the stages of sexual rest, persist in the ground. A germination of oospores takes place when the oospores encounter released from seeds or seedlings chemicals. Once germinated, the oospores can either produce a germ tube that directly infects the plant, or they form a sporangium that releases zoospores that infect the plant. A zoospore releasing sporangium represents the asexual stage in the life cycle. If water is present, the zoospores can move through the soil, which is why moisture is necessary for the pathogen to occur. As soon as the zoospores reach the roots or the seeds, they encyst , germinate and infect the plant through a germ tube. Once the infection is manifested, the pathogen's hyphae grow both inside and outside the plant, releasing enzymes that destroy the plant tissue. This destruction releases nutrients for the pathogen, also known as necrotrophy . If the plant dies, more sporangia can be formed and zoospores can be released, and the life cycle can repeat itself. Alternatively, the hyphae can continue to grow in the dead plant material and form haploid “male” ( Antheridium ) and “female” ( Oogon ) mating structures. The antheridia finally transfer their genetic material to the oogonia ("fertilization") and form diploid oospores, which overwinter and the infection can continue in the following spring.
Combat
Since Pythium irregular requires very specific environmental conditions, influencing these conditions is the first approach to combating the disease. Because zoospores need water to spread, avoiding standing water reduces the chances of the disease breaking out. In addition, excess water leads to an increase in insects that eat the roots and thus make it easier for the pathogen to infect. Water levels can be controlled when crops are not grown in areas with poor drainage, or when irrigation is controlled so that watering does not occur. Because the oospores of P. irregulare can survive even under harsh conditions, hygiene measures to limit the spread of the pathogen are extremely important. Contaminated irrigation systems, tools and seeds can spread the disease, so disinfection with heat or chemicals as well as trading in certified cleaned seeds is necessary to prevent the spread. In addition, in greenhouses it is necessary to clean the floor, work surfaces and equipment with heat or chemicals. It is also important to avoid overfertilizing the plants, as fertilizers can suppress plant defenses and damage roots, which would make infection easier for P. irregulare . Finally, in the case of previous infections with Pythium irregulare, it can be helpful to add fungicides to the soil , even if this is more suitable for greenhouse crops. It is important to plan the use of the fungicides on a rotating basis in order not to let the pathogen become resistant to one of the fungicides. Suitable fungicides include metalaxyl (mefenoxam), etridiazole and fosetyl-Al, an organic phosphorus compound. In addition, various biological "agents" such as Trichoderma harzianum and Gliocladium virens , bacteria that are generally used for biological pest control, can also be used against P. irregulare ; however, these are also more likely to be used in the greenhouse because the preparations have to be introduced into the soil. A crop rotation regime is not necessarily a good option for controlling P. irregulare because its host range is so broad and the oospores can survive in the soil for several years. In addition, the pathogen can survive well in the organic matter in the soil . But changing cultures, especially with non-host species, can reduce the amount of pathogens and reduce the risk of infection over the years.
Individual evidence
- ↑ a b c d e f g h i j k l m n o p q r s t u v Melanie Katawczik: Pythium irregulare . North Carolina State University, College of Agriculture and Life Sciences, Department of Plant Pathology. Retrieved December 6, 2016.
- ↑ a b Howard S. Judelson, Flavio A. Blanco: The spores of Phytophthora: weapons of the plant destroyer . In: Nature Reviews Microbiology . 3, No. 1, January 1, 2005, ISSN 1740-1526 , pp. 47-58. doi : 10.1038 / nrmicro1064 .
- ↑ a b c d e f Gary Moorman, Sara May: Disease Caused by Pythium . Penn State, College of Agricultural Sciences, Department of Plant Pathology and Environmental Microbiology. Retrieved December 6, 2016.
- ↑ a b c d Pythium . Penn State College of Agricultural Sciences Extension. Retrieved December 6, 2016.
- ^ A b Heffer Link, Mary Powelson, Kenneth Johnson: Oomycetes . The American Phytopathological Society. Retrieved December 6, 2016.
- ↑ a b A. J. Van Der Plaats-Niterink: Monograph of the genus Pythium . Baarn. Centraalbureau voor Schimmelcultures. P. 242. 1981. Retrieved on August 5, 2019: "with some new species added by Moorman"
- ↑ a b Pythium Genome Database . Michigan State University. Retrieved December 6, 2016.
- ↑ a b c d e f g h i j k l m n o p q Janna Beckerman: Pythium Root Rot of Herbaceous Plants . Purdue Agricultural Communication. Retrieved December 5, 2016.
- ↑ a b c d e f g h i Pythium Root Rot . University of California Agriculture and Natural Resources. Retrieved December 6, 2016.
- ^ Disease Alert: Pythium . Syngenta. Retrieved December 5, 2016.
- ^ Margaret McGrath: Managing Plant Diseases With Crop Rotation . Sustainable Agriculture Research and Education. Retrieved December 6, 2016.