Phytophthora spp. (Citrus)
|Scientific Name||Phytophthora citrophthora (R.E. Sm.& E.H.Sm.) Leonian
Phytophthora nicotianae Breda de Haan
Phytophthora imperfecta var. citrophthora (R.E. Sm.&E.H.Sm.) Sarej.
Pythiacystis citrophthora R.E. Sm.& E.H.Sm.
Phytophthora parasitica var. nicotianae (Breda de Haan) Tucker
Phytophthora imperfecta var. nicotianae (Breda de Haan) Sarej.
Phytophthora nicotianae var. parasitica (Dastur) G. M. Waterh.
Phytophthora parasitica Dastur
Phytophthora parasitica var. piperina Dastur
Phytophthora parasitica var. rhei G. H. Godfrey
|Common Names||English: Citrus gummosis, foot rot, brown rot of citrus fruit; French: Gommose des citrus, mildiou des agrumes, pourriture brune des agrumes; German: Braunfäule, Wurzelfäule, Gummose; Spanish: Pudricion parda de los citricos|
Phytophthora spp. are oomycetes grouped into the kingdom Chromista. Both P. citrophthora and P. parasitica are soil-borne fungi. They complete their life cycles in the soil.
Foot rot or gummosis is caused by two Phytophthora spp.. Symptom development often begins near the soil line; dark water-soaked areas are formed in the bark and a sour smell may occur in wet conditions. Water-soluble gum exudes from affected tissues and is particularly noticeable in dry weather. Longitudinal cracking of bark, accompanied by profuse gumming, usually is positive evidence of infection. Soil removal around affected trees reveals bark that is water soaked, slimy, reddish-brown, or black in late stages. Brown necrotic areas can be seen extending to the cambium and wood. Advanced stages of infection will result in yellow, sparse foliage.
At later stages, the dead bark dries, shrinks and cracks and patches may fall off, leaving an open canker. Trees may later collapse and die due to the girdling action of the fungal infection. Infected fruits develop a soft brown rot eventually with a characteristic pungent odor. P. citrophthora thrives under cooler conditions with seasonal rainfall and favor Mediterranean climates. It attacks aerial parts of the trunk and major limbs.
Under suitable environmental conditions, chlamydospores germinate and produce vegetative hyphae or sporangia which initiate asexual reproduction. Sporangia are also formed by mycelial hyphae in the presence of water (flooding), which is essential for this process. Both pathogens produce papillate sporangia. Sporangia of P. citrophthora are extremely variable in shape; ellipsoid, broadly ovoid, globose, limoniform or extremely distorted (27-65 x 45-90 µm). Sporangia of P. nicotianae are predominantly spherical and average 30-40 x 38-50 µm. In water, sporangia are usually borne singly or in very loose sympodia of 2-4 sporangia. Zoospores are produced inside these sporangia, and are liberated via the sporangial papilla which dissolves. The reniform zoospores are highly motile.
They then round up and encyst, casting off their flagellae. After attachment they germinate and infect cortical root tissue. Oospores are produced only by P. nicotianae which is heterothallic. They are spherical, pale yellow when mature, 24 µm in diameter, with a wall 2 µm thick, and contain irregularly shaped subcellular inclusions. The frequency with which oospores are formed under field conditions and their role in survival and disease transmission is unknown.
The vegetative mycelium is the dominant phase and produces thick-walled, resistant chlamydospores with subcellular inclusions (mainly fat and oil droplets), delimited from the somatic mycelium by a septum, becoming yellow-brown and refractile with age. Chlamydospores persisting in the soil are the primary inoculum source.
The pathogens are frequently recovered from effluent-holding ponds in nurseries that practice recirculation of irrigation run-off water. P. citrophthora occurs in soil to a depth of more than 1 m. Seasonal fluctuations in inoculum density appear to be related to soil temperature and soil water matrix potential. They can also be transported in irrigation canals such as those supplying citrus groves and may be introduced to clean land on stock infected in the nursery via infected seed. The testas of seed from infected citrus fruit carry the two pathogens.
Additional Crop Information
Species of Citrus are the most important economic hosts of P. citrophthora.
Gummosis caused by P. nicotianae occurs in all citrus-growing areas, but is more frequent in the humid tropics. Disease incidence is especially high in trees established with the graft union at or below the soil surface, exposing susceptible scion tissue to the two pathogens. Severe losses also can occur in groves subjected to flood irrigation if trees are planted on susceptible rootstocks.
Useful non-chemical contribution to Integrated Weed Management
Exposure of seeds to hot water at 48.9°C for 4-10 minutes eradicates seedborne infection. Electrolytically generated chlorine injected into citrus micro-irrigation systems effectively killed propagules of P. citrophthora. Resistant rootstocks have been used to control root, foot and stem infections on citrus caused by P. citrophthora. Citrus cultivars and related species which exhibit resistance are used in breeding programmes to produce hybrid rootstocks.
The disease can be reduced by the removal of infected or dead trees, annual inspection down to the first lateral roots, planting trees high and avoiding wounding, especially near the trunk base. Ensure that furrow irrigation water does not come into contact with the trunk by banking the soil up to a distance of 50 cm from and around the trunk base.
In young citrus plantings the majority of foot rot problems can be solved by cultural and hygiene measures which are able to suppress Phytophthora diseases. In commercial citriculture, formerly the only way to prevent or cure Pyhtophthora-induced diseases such as gummosis or foot rot on the tree trunk was the procedure to surgically remove the diseased bark and paint the infection site with a copper fungicide or captafol.
With the availability of two systemic fungicides, metalaxyl and fosetyl-Al, more effective and less laborious tools have become available for the control of Phytophthora-infections. Copper fungicides or fosetly-Al applied via foliar spray prior to seasonal rainfalls are principally effective against brown rot (pre- and post harvest) and canopy blight. A combination of foliar applications of fosetyl-Al with a soil drench of the other systemic, metalaxyl, has been shown to be even more effective. The most effective control of brown rot often entails both preharvest sprays, postharvest dip treatments and/or the use of impregnated wrappers.
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