In 1870, Lebert and Cohn first identified Phytophthora cactorum on cacti (Erwin, D.C. and O. K. Ribeiro, 1996). However, P. cactorum was first reported as the cause of crown rot of strawberry (F. Â·ananassa Dutch.) in 1952 in Germany (Deutschmann, 1954). It has since become an important disease in most European countries and can be a limiting factor to successful strawberry production worldwide (Maas, 1998).
P. cactorumÂ belongs to the class of Oomycete which is member of the kingdom Stramenopila and have a close relationship with photosynthetic organisms such as brown algae and diatoms (Sogin and Silberman 1998; Baldauf et al 2000). The class Oomycete comprises a diverse group of organisms including pathogens of plants, insects, fish and other animals. Plant-pathogenic Oomycetes cause devastating diseases in numerous crop, ornamental, and native plants.
The Oomycetes were formerly considered as members of the fungal kingdom, from which they have recently been excluded because of their cytological and biochemical characteristics. (Causin R et al, 2005). The cell walls of Oomycetes are mainly composed of 1,3-glucan polymers and cellulose; unlike fungal cell walls, they contain little chitin .Other singular features include the complex life history, alternative mechanism of pathogenicity, and complex sexual and asexual phases ( Erwin and Ribeiro 1996). However, their hyphal growth and variety of spores are morphologically and physiologically similar with fungi. Due to behavioural similarities to the true fungi (Cooke et al., 2000a), they are still on occasion referred to as fungi.
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P. cactorum is homothallic and produces oospores (resting spores) in diseased plant tissue, which makes the pathogen able to survive in the soil for many years in the absence of host plants (Sneh & McIntosh, 1974; Meyer & Schönbeck, 1975). When environmental conditions become suitable (high soil moisture, soil temperature > c. 10 Â°C) the resting spores germinate by forming sporangia which release motile, biflagellate zoospores into the soil water. Up to 50 zoospores may be produced inside one sporangium. These zoospores are chemotactically attracted by young fine root tips. After penetrating the exoderm (or the periderm in suberized fine roots), Phytophthora is growing inter- and intracellular inside the fine root with typical coralloid to irregular, non-septate hyphae .Under the proper environmental conditions, the disease can spread very quickly. A wetness period (free water on fruit surface) of one hour is sufficient for infection. The optimum temperatures for infection are between 62 and 77 degrees F (17-25 degrees C). In the case of nutrient depletion, competition by secondary antagonistic fungi or strong defense reactions by the root the Phytophthora hyphae are forming resting spores. After decomposition of the root by saprophytic fungi the resting spores are set free into the soil environment, and the cycle starts again.
Crown rot disease
Phytophthora cactorum is an important plant pathogen that can cause serious damage in agricultural and ornamental crops as well as in a wide range of forest. Although P. cactorum can parasitize a wide range of plant species, pathogenicity may vary widely across hosts.Â This pathogen can cause a number of symptoms and diseases depending on the host.Â Symptoms can include: root and collar rots, fruit rots, cankers, leaf blights, wilts, and seedling blights.Â In strawberry, P. cactorum causes crown and root rot, and can also cause leather rot of the fruit as well( Fig).Â Infection of the crown by P. cactorum can cause rot and dysfunction of the vascular system (Louws, F.J.Â 2004.).Â Symptoms typically develop during early-mid summer.Â Young leaves turn bluish and will often wilt suddenly.Â Wilting will quickly spread throughout the plant, leading to plant death.Â The crown may break at the upper end, and necrosis occurs throughout.Â Brown discoloration of the crown is diagnostic for this disease, and in most cases, symptoms appear first at the upper part and spread downward.( Seemuller, D.B.Â 1998)Â
Leather rot can infect berries at any stage of development. On green berries, diseased areas may be dark brown or natural green outlined by a brown margin. As the rot spreads, the entire berry becomes brown, maintains a rough texture, and appears leathery. The disease is more difficult to detect on ripe fruit. On fully mature berries, infection may result in little color change or discoloration ranging from brown to dark purple. Infected ripe fruit are usually softer to the touch than healthy fruit. When diseased berries are cut crosswise, a marked darkening of the water-conducting system to each seed can be observed. In later stages of decay, mature fruits also become tough and leathery. Occasionally, a white moldy growth can be observed on the surface of infected fruit. In time, infected fruit dry up to form stiff, shriveled mummies. Berries that are affected by leather rot have a distinctive, unpleasant odor and taste. Even the healthy tissue on a slightly rotted berry is bitter.
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P. cactorum has been a serious problem for strawberry production worldwide in low-lying or wet field conditions (Erwin and Ribeiro, 1996). It has wide host range (Erwinand Ribeiro, 1996); and can be found on every continent except the Polar Regions. P. cactorum can attack up to 160 herbaceous and woody plant species belonging to 60 families and 150 genera (Tucker, 1933; Nienhaus, 1960).
In Norway, the severity of the disease has varied between fields (Stensvand et al., 1999). In one of the most severe cases, 40% of the plants in a field wilted and died during one season (Stensvand et al., 1999).At present, the disease has been found in more than 50 different locations. Distributing infected plant materials, poor drainage in the soil, or surface water and infected soil on machinery, tools, etc. stimulate the dissemination of the disease. Perhaps the most important factor affecting the management of the disease is cultivar susceptibility (Seemüller, 1988; Eikemo et al., 2000; Harris & Stickels, 1981). Many of the most commonly grown strawberry cultivars in Europe are susceptible to P. cactorum (Eikemo et al., 2003) and this enhances the spreading of the disease and the severity of the disease outbreaks. For example Infection of susceptible cultivars by P. cactorum in California production systems can reduce fruit yield for susceptible cultivars by more than 50% (Browne et al..2002; Shaw and Larson, 2000)
In Norway, P. cactorum has been isolated mainly from susceptible cultivars, and has never been found in the more resistant cultivars, such as Senga Sengana or Bounty (Stensvand, Herrero & Talgø, 1999). Similarly, in Finland, P. cactorum was isolated only from field-grown strawberries of the susceptible cvs Jonsok, Zephyr and Mari (Parikka, 1991). Recently, many cultivars, both from Norwegian and foreign breeding programmes, have been released in Norway, and several of them are very susceptible to the disease (Eikemo et al., 1999). This implies that rise of crown rot in Norway seems to be associated with a major shift towards growing more susceptible cultivars than in the past.
Control of P. cactorum
, an integrated approach is most effective to reduce damages caused by P. cactorum pathogen (1, 6, 8).Â Prevention and sanitation are extremely important as this pathogen is most-often introduced through infected propagative material (transplants, plugs, cuttings, etc.).Â Cultural control may be beneficial such as adequate soil drainage and low soil pH may help to reduce disease.Â However, There are few means of eradicating it once a field has become infested, and even with fumigation, this pathogen is rarely eliminated (Sneh and McIntosh, 1974; Wilhelm and Paulus, 1980) Forexapmle in Norway a standard of one to three yearly applications of fosetyl-aluminium has become common practice for many strawberry growers (Eikemo et al, 2000). However, this fungicide rarely eliminated once a field has become infested, (Sneh and McIntosh, 1974; Wilhelm and Paulus, 1980) and therefore almost impossible to eliminate all sources of infection in strawberry nurseries (Fennimore et al., 2008
Hence, breeding resistance cultivar is the only strategy for managing the risk of crown rot caused by P. cactorum in strawberry. Differences in susceptibility to this pathogen among strawberry genotypes have been evaluated by Harrison et al (1998 ) on testing wild octoploid strawberry ( F.virginiana and F. chiloensis) and by Parikka (1998) on testing wild diploid fragaria genotypes among a wide selection of cultivated strawberry ( F.xananassa) suggested that genetic resistance can contribute to managing the consequences of this disease. Until now, little emphasis has been placed on crown rot in the Norwegian breeding programme (Eikemo, etal 2000) .
Resistance to the crown rot pathotype of P. cactorum is believed to be inherited as a polygenic character (Lerceteau-Köler et al., 2002), forexample . Shaw et al. (2006, 2008) and Denoyes-Rothan et al. (2004) found that five putative quantitative trait loci for resistance in an experimental F. xananassa population. Focusing on a simpler system than the octoploid strawberry, e.g., a diploid model system thus appears attractive to get an understanding of the nature and inheritance of the Phytophthora crown rot resistance.
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