When Jon Eisenback, professor of plant nematology at Virginia Tech’s College of Agriculture and Life Sciences, conducted nematode surveys on vegetables and rice in Cambodia this past August, one of the most surprising things he encountered in the vegetable fields was, in a word, nothing.
“One of the biggest finds from that trip was almost completely sterile soil,” Eisenback said of the surveys he and postdoctoral associate Paulo Viera conducted in vegetable farms near Siem Reap.
They visited farms growing cucumbers, sweet melons, eggplants, tomatoes, and cantaloupes to assess whether any of them were suffering from nematode invasions, but they found that all the crops were grown under plastic with drip irrigation. They had been covered with so many pesticides that there was nearly nothing living – the soil was essentially ruined.
Jon Eisenback, second from right, professor of plant nematology at Virginia Tech’s College of Agriculture and Life Sciences, testing soil in the fields of Cambodia with postdoctoral associate Paulo Viera, second from left.
Plant parasitic nematodes are microscopic roundworms that cause significant damage to many crops. In Cambodia, a country with nearly half of its labor force in agriculture, nematodes can create big problems for food production.
Because of our program’s focus on biocontrol and biopesticides to alleviate agricultural pest problems, Eisenback said that the vegetable IPM project would increase chances that vegetable farmers in Cambodia would stop the soil-killing overuse of pesticides.
After surveying the vegetable fields in the north, Eisenback and Viera traveled to the south of the country to conduct nematode surveys on rice. Given the dearth of scientific literature published on nematodes in Cambodia relating to rice, Eisenback and Viera weren’t sure what to expect. However, they found that the rice fields they surveyed showed a significant loss of production caused by the rice root nematode.
“Every root we looked at had lesions,” Eisenback said. The culprit was a parasitic nematode called Hirschmanniella mucronata. “Rice roots should be creamy white. These were speckled with brown and orange lesions.”
Eisenback expects that these nematodes could cause a 20 to 30 percent crop loss of rice in affected fields.
The next step is field demonstrations; to undertake them, half the fields should be treated with nematicide to measure the effect. Eisenback also said he hopes to continue the survey to see what other nematodes are there.
“I would suspect that there are other fields with other nematode problems.”
As for the vegetable fields with the sterile soil near Siem Reap, Eisenback offered a recommendation for them as well: Don’t use so many toxic pesticides.
With IPM IL’s projects up and running in the region, that should soon become less of a problem.
A juvenile root-knot nematode, Meloidogyne incognita, penetrates a tomato root. Once inside, the juvenile, which also attacks cotton roots, causes a gall to form and robs the plant of nutrients. Photo by William Wergin and Richard Sayre. Colorized by Stephen Ausmus.
19th Biennial Group Meeting of the “All India Coordinated Research Project (AICRP) on Nematodes in Cropping Systems”
At the 19th Biennial Group Meeting of the “All India Coordinated Research Project (AICRP) on Nematodes in Cropping Systems” recently held at University of Agricultural and Horticultural Sciences, Shivamogga (Karnataka) India; experts from the country conveyed that an aggressive (with high reproduction rate, more damage to host plants and wide host range) root knot nematode, Meloidogyne enterolobii, got introduced and established through guava root stocks from Chhattisgarh, is causing huge losses in Dindigul, Coimbatore, Villupuram, Dharampuri and Krishnagiri districts of Tamil Nadu. The group emphasized that there is an urgent need to strengthen and enforce domestic quarantine mechanism to suspend spread of plant parasitic nematodes with vegetative propagules, especially through seed potatoes and rooted plants – along with soil, from nurseries/ sick plots/ hot-spot areas to disease free niches. In their opinion, presently nurseries in the country are having a field day and incorrigible for spreading pests without meeting any cleanliness standards or phytosanitary regulations. To break the pathway, it was suggested to enforce registration and licensing of plants and horticultural nurseries.
The recommendation from the Biennial Workshop is immensely important for reducing crop losses of horticultural crops in the country. Horticulture plant nurseries are extremely complex agricultural systems, recorded as pathways for several pests and diseases. Dr. Rajan said that the situation has become further cumbersome with ‘on line’ availability and sale of live ornamental and horticultural plants in the country. As disease management in nurseries/ green houses require specialisation; nematologists from the group ventured a draft road map – with details of detection, exclusion, risk analysis, critical control points for nursery stocks, infrastructure required for prophylactic measures, and costs involved for a prophylactic holistic system approach for registration/ certification for Nurseries and Green Houses.
In the address, Dr. D. J. Patel (Former Dean, Anand Agriculture University) and Dr. P. P. Reddy (Former Director, Indian Institute of Horticulture Research), well known experts in the subject expressed deep concerns about new nematode diseases in pomegranate, guava, coconut, banana, spices and vegetables all over the country through propagules. There is urgent need for policy support from Indian Council of Agricultural Research (ICAR), Department of Agriculture and Cooperation as well as Horticulture Mission for framing mandatory regulatory provisions for registration, licensing and certification of protected cultivation houses, nurseries and green houses especially for pest / quarantine requirements.
Dr. R. K. Walia, Project Coordinator (Nematodes), presented a brief history, background and the salient achievements of the AICRP on nematodes and overall scenario Plant Nematology research in India. He expressed serious concerns about the losses in crops due to nematode diseases and urged upon the nematologists to devise integrated approaches to manage root knot nematode (Meloidogyne spp.) problem in recently established poly-houses (for promoting cultivation of vegetables and ornamental) all over the country.
New publications “Pictorial guide on important nematode diseases of Karnataka”, “Comprehensive monograph of rice root-knot nematode (Meloidogyne graminicola)”, “Status of plant nematode diseases in Karnataka – a review”, and “Compendium of new plant parasitic nematode diseases of Karnataka”, along with a number of bulletins on serious issues were also launched on the occasion.
A juvenile root-knot nematode penetrates a tomato root. Photo by William Wergin and Richard Sayre. Colorized by Stephen Ausmus.
Nematodes, or roundworms, are the most numerous animals on earth, parasitizing most plants and animals. These infections are responsible for many of the most common neglected tropical diseases, causing significant morbidity and mortality. Soil-dwelling nematodes also put food security at risk by attacking agriculturally important plants. While the human response to dealing with parasite infection has been thoroughly researched; it has only recently been discovered that plant-parasitic nematodes not only activate defensive responses in plants, but also provide nematode-mediated immunity to subsequent attack by pathogens and viruses.
Despite more than 4,100 species of plant-parasitic roundworms being identified, two major groups of nematodes are responsible for most agricultural plant damage. The root-knot nematodes, Meloidogyne spp. damage plants by producing galls on roots; whereas, the cyst nematodes, Heterodera and Globodera spp., by the formation of root cysts.
Soil-dwelling nematodes are ubiquitous and rich arable soil may contain up to 3 billion worms per acre. It therefore comes as no surprise that infected crops result in over $100 billion worth of agricultural damage globally per annum. For British crops alone, damage by cyst nematodes, Globodera rostochiensis and G. pallida, account for an estimated £50 million damage each year.
Nematode control therefore is a serious business; however, following the current tightening of legislation, withdrawal from use of inorganic pesticides (the primary source of pest and disease management over the past decades) and a lack of resistant plant varieties, there is an urgent need to understand more about plant natural defenses to promote resistance to nematodes and other invaders.
Plant defenses against pathogens
Plant defenses can be broadly grouped into constitutive (continuous) defenses and inducible defenses. Toxic chemicals or defense-related proteins are typically only produced after pathogens are detected due to the high energy costs associated with their production and maintenance. To allow detection of, and rapid response to, potentially harmful pathogens plants have evolved several layers of highly developed surveillance mechanisms to try and circumvent serious damage.
The first line of defense consists of inducible defenses, which are mounted when plant cells recognize microbe-associated molecular patterns (MAMPs), such as lipopolysaccharides, flagellin, peptidoglycan and other compounds commonly found in microbes. Plant cells then become fortified against attack, conferring protection from the invading pathogen. Although MAMP’s have been well characterized, up until recently, it remained unclear as to whether plants could detect conserved molecular patterns derived from plant-parasitic animals, such as soil-dwelling nematodes.
‘Nematode-associated molecular patterns’
A number of studies have previously shown that, in response to plant-parasitic nematode infection, plants quickly activate defense pathways similar to those induced by other pathogens. Although these findings were very promising, what the nematode-derived signals actually were remained a mystery.
Following the discovery that non-parasitic soil nematodes can also induce plant defenses, a conserved nematode signature molecule appeared to be a likely trigger for activating the plant defense response. Ascarosides are pheromones exclusive to nematodes that are used to regulate development and social behaviours. Ascarosides represent an evolutionarily conserved family of signalling molecules, of which more than 200 different ascaroside structures from over 20 different species have been identified. Due to the highly conserved nature of these molecules, it seemed plausible that plant hosts and nematode-associated microorganisms may have evolved the means to detect and respond to this ancient nematode molecule. A recent study published in Nature Communications investigated whether ascarosides can be detected by plants, and whether detection of the molecules induced the plant-defence response.
Profiles of ascarosides from adult and juvenile stages of a number of agriculturally relevant species of plant-parasitic nematodes were characterised using mass spectrometry (MS) to analyse the metabolome excreted into media supernatant. MS analysis of exo-metabolome samples revealed excretion of similar sets of ascarosides in all analysed species; however, ascr#18 was identified in all plant-parasitic nematodes as the most abundant molecule.
In order to determine whether ascr#18 could be perceived by plants and influence plant-defensive responses to different pathogens, the ascaroside was applied in various concentrations to Arabidopsis roots 24 h prior to leaf innoculation with pathogens. By monitoring expression of MAMP-triggered immunity (MTI) markers and defense-related genes in leaves at different time points after root treatment with ascr#18, characteristic defense responses such as MAMP-triggered immunity were shown to be induced. Interestingly, local and systemic defenses were also shown to be activated by ascr#18 application to leaves.
Detection of ascr#18 by plants increased resistance to viral, bacterial, oomycete, fungal and nematode infections in Arabidopsis, as well as tomato, potato and barley. Additionally, three other ascarosides applied to different plants showed defense responses were induced by structurally diverse ascarosides, but that this varied in a structure- and species-dependent manner.
Plant crops suffer considerable damage every year from parasites and pathogens, putting food security at great risk. The ability to activate plant immune responses as and when required by using signalling molecules, such as ascarosides, is an exciting discovery that could contribute to improving the economic and environmental sustainability of agriculture. One potential application could be spraying of ascr#18 on crop leaves; as although plants primarily encounter ascarosides via their roots, leaf exposure to low ascr#18 concentrations was also effective at inducing the defense responses to confer fortification against attack.
Partnering for Innovation
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With foreign markets requiring reductions in the use of chemicals, there is great demand for biological solutions to pest management. Zamorano University will promote the use of beneficial nematodes instead of traditional pesticides to control insect infestations in select horticultural crops. Through scale-up of their biocontrol laboratory, Zamorano will produce and sell 20 times as many doses of beneficial nematodes over the previous year. Small producers will be able to access this biocontrol at a much lower cost than a synthetic chemical product throughout the western departments of Honduras.
Outcome: 9,000 hectares of smallholder land will use biological pest control. In addition, Zamorano will sell through a commercial partner to build a sustainable distribution channel in the country.
A recent outbreak of Ufra rice disease is decimating rice crops in the Ayeyarwady region. Photo: Hong Sar/Mizzima
A recent outbreak of Ufra rice disease is decimating rice crops in Ayeyarwady Region. Caused by a tiny nematode (Ditylenchus angustus) that feeds on the inner part of unmerged rice leaves, the disease initially causes the discoloration of rice plants. Infected plants have panicles with many unfilled grains, lowering yields, and in severe cases, causing complete crop failure.
In 2002, it was estimated that 15 percent of Myanmar’s rice farmers were affected by the disease, but because of improper containment strategies, the number is now thought to be higher. For example, in Ayeyarwady Region’s Myaungmya district, up to 68 percent of farmers are thought to be affected.
This year the monsoon has unleashed widespread infestation throughout Ayeyarwady Region, particularly in Pathein district. In many cases, farmers affected by the disease are only able to harvest up to 20 baskets of rice for each quarter acre, compared to an average of more than 70 baskets in a good year. At Hay Man Village in Bogele Township, 30 acres out of 55 acres were rendered unproductive by the disease.
The spread of the disease is blamed on inadequate understanding. Simple measures, such as burning crop residue, planting short-life rice varieties and proper water management, are enough to contain and, eventually, eradicate Ufra. Unfortunately, Myanmar farmers often lack enough knowledge about these measures. They also lack the tools needed to diagnose the disease from among a variety of pests and diseases likely to affect their crops. This means that they are trying to salvage their crops by using costly pesticides that are not as effective as the preventive measures outlined above.
The Myanmar Agriculture Service does what it can to assist but does not have the funds to make frequent village visits to help farmers diagnose and respond effectively to outbreaks of Ufra. Another challenge is that insufficient resources have been allocated to building the embankments, floodgates and other basic infrastructure that would significantly help farmers to control the disease. During a joint Proximity Design-MAS visit to Paw De Kaw Village in Naputa Township on September 7, 2014, it was revealed that more than 50 of the area’s farmers would be unable to implement the suggested prevention measures because their land was too close to a nearby river to allow for proper water management. In Paw De Kaw Township, floodgates are all that are needed to help control the spread of Ufra but the area’s farmers have suffered years of poor harvests and cannot afford to build them without government support.
Because the nematode responsible for Ufra needs high humidity, the disease thrives in South Asia, especially during years of heavy rain and flooding. Though cases of the disease have been documented in parts of India, Vietnam, Malaysia, Myanmar, and Thailand, it’s been most problematic in neighbouring Bangladesh. Even there, however, progress has been achieved by encouraging farmers to plant lowland rice varieties. Meanwhile, farmers in lower Myanmar, such as the residents of Paw De Kaw Village, continue to struggle.
This Article first appeared in the October 30, 2014 edition of Mizzima Business Weekly.
Mizzima Business Weekly is available in print in Yangon through Innwa Bookstore and through online subscription at http://www.mzineplus.com
A systematic survey for the presence of potato cyst nematodes (_Globodera rostochiensis_ and _G. pallida_ — both EPPO A2 List) was initiated in Bosnia and Herzegovina in 2011. Until 2012, only _G. rostochiensis_ had been detected in Bosnia and Herzegovina.
In autumn 2012, viable cysts were found in 2 soil samples originating from 1 field located 70 km [about 43 miles] east of Sarajevo.
Morphological and molecular analysis confirmed the occurrence of _G. pallida_ in these samples. More samples were collected from the other fields of the grower concerned, as well as from their surroundings, but no cysts were found in these additional samples.
A more intensive sampling regime was implemented in the infested field (1.1 ha [2.7 acres]) and revealed a high infestation of 1 cyst per gram of soil in the infestation focus. The high infestation level and the use of farm-saved seed potatoes by the grower suggest that the introduction of _G. pallida_ probably took place several years before via imports of infected seed potatoes.
Phytosanitary measures were taken on the infested field (prohibition to grow potatoes for the next 6 years, continuing sampling).
[Both golden (_Globodera rostochiensis_, with at least 5 races) and pale (_G. pallida_) potato cyst nematodes (PCNs) cause serious crop losses in potato. Other solanaceous crops (such as tomato) and weeds may serve as pathogen reservoirs. PCN symptoms on potato include stunting, yellowing, and wilting of leaves as well as a reduced root system. PCNs may lead to complete crop failure. Diseased plants first occur in isolated patches and these become larger with each new crop.
The nematodes can survive in soil for up to 20 years as cysts. Spread occurs via infected soil, water, wind, or on plant material (such as the seed potatoes suspected above). Disease management includes exclusion, long crop rotation with non-host species, use of crop cultivars resistant to specific PCN races and nematicides. These control measures can be combined to keep nematode levels below economic thresholds. Both PCNs have been included on the quarantine lists of the European Plant Protection Organisation (EPPO).
In the Eurasian area, golden PCN is widespread but pale PCN has a more restricted distribution and its detection in specific areas is considered of significance to the respective region. It would be important to ascertain the original source of the infection in Bosnia and Herzegovina, as that location would also require appropriate measures to improve the health of local solanaceous crops.
Growers are reminded to investigate uneven crops to check if plant parasitic nematodes are present, after root lesion and burrowing nematodes were found in the Moora area.
Roots of barley crop impacted by P. penetrans
Department of Agriculture and Food nematologist Sarah Collins recently visited the area where a number of wheat and barley crops have been damaged by burrowing (Radopholus) nematodes and by root lesion nematodes (Pratylenchus, RLN).
Samples were diagnosed by AGWEST Plant Laboratories as the RLN species Pratylenchus penetrans.
“This was unexpected as this species is more often associated with cropping in the cooler growing areas but we have had a number of P. penetrans diagnoses this season from locations across areas of the Wheatbelt including Moora, Northam and Wagin,” Dr Collins said. “It is one of the species that we know less about.
“The ‘burrowing nematode’ detected in a nearby barley crop is also interesting as crop damage has not been reported for this plant parasitic nematode for a number of years.”
Correct identification of nematodes is important because the choice of suitable break crops to mitigate future damage is dependent on knowing which plant parasitic nematode species are present.
“It is possible that consecutive seasons favourable to RLN and burrowing nematode and the increasing inclusion of canola to crop rotations may be contributing to the build-up of nematode numbers,” Dr Collins said.
The reports follow survey work by the Focus Paddocks project, supported by Grains Research and Development Corporation, which has detected increasing levels of RLN across 184 paddocks surveyed since 2010.
“We are seeing the highest prevalence of RLN populations in at least a decade,” Dr Collins said.
Above ground symptoms of plants infected with root lesion and burrowing nematodes include stunting, poor growth, early wilting, premature yellowing of lower leaves and dying back from the tips.
Below ground symptoms often include reduced root systems with fewer lateral roots and root hairs compared to nearby healthy plants. Brown/dark coloured lesions along the roots may also be seen.
Suspected root disease or nematode problems in-crop can be confirmed by a chargeable laboratory analysis of soil and/or roots by AGWEST Plant Labs, which are contactable on 9368 3721 or firstname.lastname@example.org.
Nematodes will be on the agenda at a plant disease ID course for agronomists and development officers run by the department in South Perth next week (19 and 20 Aug). The department can deliver courses on root disease identification for grower groups at their local centre. To enquire about these courses contact Dominie Wright at the department on 9368 3875.