| Taro leaf blight | |
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| Phytophthora colocasiae sporangia on sporangiophores | |
Scientific classification  | |
| Domain: | Eukaryota |
| Clade: | Diaphoretickes |
| Clade: | SAR |
| Clade: | Stramenopiles |
| Phylum: | Oomycota |
| Order: | Peronosporales |
| Family: | Peronosporaceae |
| Genus: | Phytophthora |
| Species: | P. colocasiae |
| Binomial name | |
| Phytophthora colocasiae Racib. 1900 | |
First described in Java by Marian Raciborski in 1900, taro leaf blight is caused by the oomycete Phytophthora colocasiae, which infects primarily Colocasia spp. and Alocasia macrorrhizos . [1] P. colocasiae primarily infects leaves, but can also infect petioles and corms. [2]
Symptoms on leaves initially occur where water droplets accumulate and eventually form small, brown spots surrounded by halos on the upper surface of leaves. [2] These spots expand very quickly and form large brown lesions. [3] The entire leaf can be destroyed within a few days of the initial appearance of symptoms under wet conditions. [4] The undersides of leaves have spots that look water-soaked or gray, and as they expand, blight forms and the leaf is destroyed within a few days. [2] Symptoms occur in a day/night pattern where water soaked areas expand during the night and then dry out during the day. As a result, additional water marks form leading to increasingly larger lesions. As the lesions expand, sporangia develop most actively at the margin of the lesion and progress to attack healthy tissue. [1]
One characteristic feature found on leaves is the formation of bright orange droplets oozing out from above and below water soaked leaf surfaces. [1] As a result, the droplets dry out during the day and become crusty. [1] Another sign of P. colocasiae infection are masses of sporangia that form a white, powdery ring around the lesion. Symptoms on petioles includes gray to brownish black lesions that can occur anywhere on the petioles. Petioles become soft and may break as the pathogen destroys the host. [1]
Symptoms on corms are often rubber-like and soft as well as having a light tan color. These symptoms occur rapidly and can arise anywhere on the corm and are often subtle in early stages. Decayed corm tissue appears brown and turns purplish in advanced stages of infection. [1]
Lesions can also be formed by sporangia that are splashed by rain. The dead central area breaks and falls out as the lesion gets larger. [1] The rate of spread for this disease is very high which results in a high percentage of yield loss. [4]
P. colocasiae is an oomycete and is thus characterized by oospores and coenocytic hyphae. [4] Oospores have very thick-walls which provide durable survival structures. As a result, oospores overwinter in soil, underground storage organs, or on leaf debris left in the field after harvest. However, inoculum does not survive for very long on leaf tissue. Other Colocasia plants such as Elephant-ear and Dasheen are an additional means of survival for this pathogen. Finally, chlamydospores have been produced under ideal laboratory conditions in culture, and may also serve as a survival structure in addition to oospores. However, chlamydospores have not yet been observed in the field. Therefore, it is not known if chlamydospores are really part of the Phytophthora colocasiae disease cycle. [5]
Upon infection, oospores that overwinter on leaf tissue and petioles give rise to sporangiophores which have lemon-shaped sporangia at their tips. Sporangia can infect taro leaves either directly via germ tubes or indirectly by producing zoospores. Whether sporangia infect directly or indirectly depends on weather conditions. [4]
If weather conditions are favorable, such as warm temperatures, sporangia infect directly via germ tubes. Germ tubes give rise to appressorium which form haustorium and allow the pathogen to extract nutrients without penetrating the host’s cell membrane. As a result, more sporangia are formed and if weather conditions remain favorable, additional sporangia are produced and infection continues either directly or indirectly on other hosts.
Indirect infections occur under unfavorable or very wet conditions by releasing zoospores from sporangia. Zoospores lose their flagella, become cysts, germinate and feed on the host via a germ tube, and produce more sporangia to continue the disease cycle. [6]
The slanted shape of the taro leaf encourages sporangia and zoospores to spread to other hosts via splash from rain. The pathogen can also be transmitted across fields by infected plant material or contaminated tools. [5] The pathogen can survive as mycelium for a few days in dead and dying plant tissues as well as in infected corms. [1] On the other hand, encysted zoospores of P. colocasiae can survive for several months without a host.
Once infection season comes to an end, sexual reproduction occurs to form an oospore. In order to have successful sexual reproduction, weather conditions must be favorable and mating types must match. Two mating types, A1 and A2, exist for Phytophthora colocasiae. Hormonal signaling allows for sporangia of the two mating types to come together and initiate the development of oogonia and antheridia. [5] An oogonium can be likened to a female reproductive organ while an antheridium carries out the role of a male reproductive organ. The penetration of an oogonium via an antheridium leads to the formation of a sexual spore or an oospore. The oospore will overwinter and germinate to produce infectious sporangia once conditions improve. [4]
The pathogen grows voraciously in areas with high humidity and heavy rainfall in addition to an optimal pH of 6.5 and temperature of 28 °C (82 °F). [7]
The aforementioned cool, wet, and humid conditions favor both the asexual and sexual reproduction of Phytophthora colocasiae. The ideal temperature range for this pathogen is 10–35 °C (50–95 °F). Sporangia, which develop most rapidly at margins of leaf lesions, can germinate directly on leaves at temperatures ranging from 20–28 °C (68–82 °F), or spread to neighboring leaves via rain splash. During less ideal conditions, such as low temperatures nearing 20 °C (68 °F) and high humidity, sporangia release zoospores for indirect germination. Germination occurs over a period of approximately two hours, followed by a 2-4 day incubation period between germ tube penetration and the onset of disease symptoms. [7]
The ideal temperatures for reproduction of the Phytophthora colocasiae pathogen has led to its distribution throughout cool tropical areas of Southeast Asia, from where the pathogen is thought to have originated. P. colocasiae has been observed in Indonesia, China, India, the Philippines, Malaysia, Hawaii, Papua New Guinea, and the British Solomon Islands. [7]
In the past, control of P. colocasiae has been aimed largely at limiting the amount of inoculum via cultural practices. An example of such a practice is roguing, which involves the removal of all or part of infected leaves. This practice has ultimately proved ineffective as the removal of leaves largely mimics the defoliating effects of the Taro Leaf Blight disease itself, and exacerbates the losses in yield already devastating the crop. [5]
Increased spacing between taro plants has been explored as a method of limiting transmission. However, this method has also proven ineffective as taro plants grow best when planted close together. Therefore, increasing space between plants also decreases yield. [5]
Chemical control of P. colocasiae has offered some relief in the form of preventative sprays containing copper, manganese, and zinc. The slanted shape of taro leaves, and prevalence of the crop in wet climates, necessitates numerous applications which may not be economically practical. The systemic fungicide metalaxyl has also been found to be effective in controlling P. colocasiae, especially when applied in conjunction with the pesticide mancozeb at the first sight of disease symptoms. [4] While chemical control is one of the more effective methods of managing this pathogen, the fact that taro is usually a subsistence crop makes chemical control economically impractical and environmentally unsustainable. [5]
A recent study obtained promising results using Eucalyptus globus essential oil to combat P. colocasiae. Upon chemical analysis of the essential oil, researchers determined the most abundant compounds to be 1,8-cineole, α-pinene, and p-cymene. These compounds make up 26.4%, 14.1% and 10.2% of the essential oil, respectively. In vitro studies demonstrated complete inhibition of sporangia germination and mycelial growth following applications of Eucalyptus globus essential oil at a concentration of 0.625 mg/mL and inhibition of zoospore germination at a concentration of 0.156 mg/mL. In situ, these compounds completely inhibited sporulation, necrosis, and overall disease symptom expression at a concentration of 3.5 mg/mL. [8]
Despite the promise of chemical control, genetic resistance currently offers the best long-term control of P. colocasiae. In 2013, researchers were able to confer resistance to the taro plant via the oxalate oxidase (OxO) gene gf2.8 of wheat ( Triticum aestivum ). The researchers were able to isolate a transgenic line which did not develop disease symptoms following inoculation with P. colocasiae zoospores. [9] Several Taro Leaf Blight-resistant varieties, such as the K333, K345, and Ainaben, have also been identified. While genetic resistance appears to be the most powerful tool for managing P. colocasiae thus far, certain genetic modifications result in changes in the taste and appearance of the taro plant. Due to its grave cultural and economic significance, the challenge of developing resistant varieties without decreasing the cultural and economic value of the crop remains. [6]
Taro is the fourteenth most consumed vegetable worldwide and is a staple crop both in the diet and economy of the tropics. Many tropical nations rely on taro as a main export. Taro Leaf Blight causes varying losses in corm yield depending on how susceptible the cultivars are to Taro Leaf Blight infection and damage. Reductions in corm yield of 25-50% have been reported in various locations across the Pacific. Losses of 25-35% of corm yield have been recorded in the Philippines while in some extreme cases, losses of 95% have been recorded in various cultivars across Hawaiʻi. [5]
The most recent epidemic of Taro Leaf Blight spanned the Samoan Archipelago from 1993-1994. Taro export made up 58% of Samoa’s economy and brought in 3.5 million US dollars annually immediately prior to the epidemic in 1993. In 1994, taro exports brought in only $60,000 US dollars - a 99.98% drop in profit in just one year’s time. [5]
The reason Taro Leaf Blight was able to ravage Samoa’s taro crop was because taro is vegetatively propagated via cuttings and not seeds. Taro monocultures with no leaf blight resistance had been created with very little genetic variance. [10] This narrow gene variance left the monocultures devoid of resistant plants to buffer the spread of the pathogen allowing it to quickly spread in the warm humid climate of Samoa, devastating taro production throughout the archipelago. Currently, the taro exportation of Samoa has rebounded due to crossing the Samoan variety with resistant varieties from Southeast Asia. [10] The implementation of virus free tissue testing has ensured that no infected vegetative tissue can be sold and grown on Samoan soil. [10] Samoa’s epidemic served as an example for other taro exporting countries to ensure Taro Leaf blight resistance among plantations and to test tissue to ensure that the disease does not spread across other countries. [5]
Phytophthora sojae is an oomycete and a soil-borne plant pathogen that causes stem and root rot of soybean. This is a prevalent disease in most soybean growing regions, and a major cause of crop loss. In wet conditions the pathogen produces zoospores that move in water and are attracted to soybean roots. Zoospores can attach to roots, germinate, and infect the plant tissues. Diseased roots develop lesions that may spread up the stem and eventually kill the entire plant. Phytophthora sojae also produces oospores that can remain dormant in the soil over the winter, or longer, and germinate when conditions are favourable. Oospores may also be spread by animals or machinery.
Aphanomyces euteiches is a water mould, or oomycete, plant pathogen responsible for the disease Aphanomyces root rot. The species Aphanomyces euteiches can infect a variety of legumes. Symptoms of the disease can differ among hosts but generally include reduced root volume and function, leading to stunting and chlorotic foliage. Aphanomyces root rot is an important agricultural disease in the United States, Europe, Australia, New Zealand, and Japan. Management includes using resistant crop varieties and having good soil drainage, as well as testing soil for the pathogen to avoid infected fields.
Phytophthora cactorum is a fungal-like plant pathogen belonging to the Oomycota phylum. It is the causal agent of root rot on rhododendron and many other species, as well as leather rot of strawberries.
Phytophthora medicaginis is an oomycete plant pathogen that causes root rot in alfalfa and chickpea. It is a major disease of these plants and is found wherever they are grown. P. medicaginis causes failure of stand establishment because of seedling death. Phytophthora medicaginis is part of a species complex with Phytophthora megasperma.
Phytophthora nicotianae or black shank is an oomycete belonging to the order Peronosporales and family Peronosporaceae.
Pythium irregulare is a soil borne oomycete plant pathogen. Oomycetes, also known as "water molds", are fungal-like protists. They are fungal-like because of their similar life cycles, but differ in that the resting stage is diploid, they have coenocytic hyphae, a larger genome, cellulose in their cell walls instead of chitin, and contain zoospores and oospores.
Phytophthora erythroseptica—also known as pink rot along with several other species of Phytophthora—is a plant pathogen. It infects potatoes causing their tubers to turn pink and damages leaves. It also infects tulips (Tulipa) damaging their leaves and shoots.
Pythium aphanidermatum is a soil borne plant pathogen. Pythium is a genus in the class Oomycetes, which are also known as water molds. Oomycetes are not true fungi, as their cell walls are made of cellulose instead of chitin, they are diploid in their vegetative state, and they form coenocytic hyphae. Also, they reproduce asexually with motile biflagelette zoospores that require water to move towards and infect a host. Sexually, they reproduce with structures called antheridia, oogonia, and oospores.
Pythium graminicola is a plant pathogen infecting cereals.
Pythium volutum is a plant pathogen infecting wheat, barley, and turfgrass. It is known to be sensitive to some of the compounds typically present in selective media commonly used for isolating Pythium spp., so isolation may require alternative methods.
Sclerophthora macrospora is a protist plant pathogen of the class Oomycota. It causes downy mildew on a vast number of cereal crops including oats, rice, maize, and wheat as well as varieties of turf grass. The common names of the diseases associated with Sclerophthora macrospora include “crazy top disease” on maize and yellow tuft disease on turf grass. The disease is present all over the world, but it is especially persistent in Europe.
Albugo is a genus of plant-parasitic oomycetes. Those are not true fungi (Eumycota), although many discussions of this organism still treat it as a fungus. The taxonomy of this genus is incomplete, but several species are plant pathogens. Albugo is one of three genera currently described in the family Albuginaceae, the taxonomy of many species is still in flux.
Plasmopara viticola, the causal agent of grapevine downy mildew, is a heterothallic oomycete that overwinters as oospores in leaf litter and soil. In the spring, oospores germinate to produce macrosporangia, which under wet condition release zoospores. Zoospores are splashed by rain into the canopy, where they swim to and infect through stomata. After 7–10 days, yellow lesions appear on foliage. During favorable weather the lesions sporulate and new secondary infections occur.
Phytophthora capsici is an oomycete plant pathogen that causes blight and fruit rot of peppers and other important commercial crops. It was first described by L. Leonian at the New Mexico State University Agricultural Experiment Station in Las Cruces in 1922 on a crop of chili peppers. In 1967, a study by M. M. Satour and E. E. Butler found 45 species of cultivated plants and weeds susceptible to P. capsici In Greek, Phytophthora capsici means "plant destroyer of capsicums". P. capsici has a wide range of hosts including members of the families Solanaceae and Cucurbitaceae as well as Fabaceae.
Phytophthora fragariae is a fungus-like (oomycete) plant pathogen that causes red stele, otherwise known as Lanarkshire disease, in strawberries. Symptoms of red stele can include a red core in the roots, wilting of leaves, reduced flowering, stunting, and bitter fruit. The pathogen is spread via zoospores swimming through water present in the soil, released from sporangia.
Phytophthora megakarya is an oomycete plant pathogen that causes black pod disease in cocoa trees in west and central Africa. This pathogen can cause detrimental loss of yield in the economically important cocoa industry, worth approximately $70 billion annually. It can damage any part of the tree, causing total yield losses which can easily reach 20-25%. A mixture of chemical and cultural controls, as well as choosing resistant plant varieties, are often necessary to control this pathogen.
Phytophthora kernoviae is a plant pathogen that mainly infects European beech and Rhododendron ponticum. It was first identified in 2003 in Cornwall, UK when scientists were surveying for the presence of Phytophthora ramorum. This made it the third new Phytophthora species to be found in the UK in a decade. It was named Phytophthora kernoviae after the ancient name for Cornwall, Kernow. It causes large stem lesions on beech and necrosis of stems and leaves of Rhododendron ponticum. It is self-fertile. It has also been isolated from Quercus robur and Liriodendron tulipifera. The original paper describing the species, stated it can infect Magnolia and Camellia species, Pieris formosa, Gevuina avellana, Michelia doltsopa and Quercus ilex. Since then many other plants have been identified as natural hosts of the pathogen. Molecular analysis has revealed that an infection on Pinus radiata, recorded in New Zealand in 1950, was caused by P. kernoviae. The pathogen was also noted on Drimys winteri, Gevuina avellana, Ilex aquifolium, Quercus ilex, Vaccinium myrtillus, Hedera helix, Podocarpus salignas.
Phytophthora quercina is a papillate homothallic soil-borne plant pathogen causing root rot of oak tree species in Europe. It is associated with necrotic fine roots.
Phytophthora hydropathica is an oomycete plant pathogen that is found in aquatic environments such as irrigation and river water. The pathogen was previously classified as P. drechsleri Dre II before being categorized as its own distinct species. P. hydropathica has been primarily found in association with ornamental plant nurseries. The pathogen has been isolated throughout the Southern United States, as well as internationally in Mexico, Italy, and Spain.
Black rot on orchids is caused by Pythium and Phytophthora species. Black rot targets a variety of orchids but Cattleya orchids are especially susceptible. Pythium ultimum and Phytophthora cactorum are known to cause black rot in orchids.
| Authority control databases: National |
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