Plants Diseases

I

INTRODUCTION

Diseases of Plants, deviations from the normal growth and development of plants incited by microorganisms, parasitic flowering plants, nematodes, viruses, or adverse environmental conditions. In the United States alone, known plant diseases attributable to these causes are estimated to number more than 25,000; the estimated annual losses therefrom add up to several billion dollars. Injuries to plant life due primarily to insects, mites, or animals other than nematodes are not regarded as plant diseases.

Benefits of Virus-Resistant Crops

Viral-resistance in crop plants is engineered based on the premise that the host plants express genes that interfere with the essential functions of the virus thereby upsetting the balance of related components. Coat-protein mediated pathogen-derived resistance is the commonly used method to introduce resistance in crop plants to viruses. In this approach, plants are transformed with a specific virus coat protein gene which interfere with critical processes such as replication, post-transcriptional gene expression, virion coating and uncoating and intercellular transport (Beachy et al., 1990; Kaniewski and Lawson, 1998). Constitutive expression of the coat protein gene confers protection against infection with the virus from which the gene is derived and possibly against infection from other related viruses (Di et al., 1996).

Commercially available virus-resistant crops include papaya, summer squash and potato. Virus-resistant papaya and squash have been available since 1998. Virus-resistant trait was stacked with Bt to broaden the range of protection against pest populations in potato and was discussed in insect-resistant plants section.

Adoption of virus-resistant papaya has been rapid since its introduction (53% in 2000). It is expected that virus-resistant papaya will be planted on almost 90% of the acreage in the next few years. On the other hand, biotechnology-derived summer squash was planted on less than 10% of the total acreage in the United States in 2000. Lack of resistance to important pathogenic viruses coupled with the availability of virus-resistance trait in only few varieties is cited to be the reasons for low adoption.

Biotechnology-derived virus-resistant crops are particularly valuable as management options that limit viral infestations to prevent serious yield losses are limited. Since viral infestations cannot be controlled by chemical means, conventional way to manage viruses is to manage their transmission by controlling insects. Preventing the spread of virus by controlling insect vectors is not effective for two reasons: virus transmission through insects is almost instantaneous which render insecticide applications futile and secondary hosts that harbour the viruses do not exhibit symptoms. Another widely used management technique to control viruses is use of resistant varieties in crops such as squash. Natural resistance may not be available to combat viruses in crops such as papaya. However, both these methods are not completely effective in preventing viral infestations.

Papaya industry in the United States concentrated mainly in Hawaii was on the brink of extinction in 1990s due to the epidemic infestations of papaya ringspot virus (PRSV). PRSV is the most important disease of papayas. The PRSV is transmitted by aphids and cannot be eradicated as secondary hosts harbour the virus without exhibiting any symptoms. Hawaiian farmers had no choice other than destroying the infested plants to contain the disease.

Viruses that limit summer squash production in the United States are zucchini mosaic virus (ZMV), watermelon mosaic virus 2 (WMV), cucumber mosaic virus (CMV) and papaya ringspot virus (PRSV). All these viruses are transmitted by aphids and affect a range of plants making it difficult and impossible to prevent virus infestations. Foliar applications of highly refined petroleum oil are widely used to serve as a barrier between aphid and the plant to prevent virus transmission. However, frequent applications are needed to ensure season-long protection.
Virus-resistant plants enable growers to reduce the use of pesticide by eliminating the need to spray insecticides to control the insects that transmit viral diseases, or herbicides to kill the weeds that harbour those insects. As a result, overall pesticide use and crop production costs have been reduced. An indirect benefit of virus-resistant crops is they do not serve as reservoirs for viruses unlike their conventional counterparts. As a result, further spread of virus to susceptible plants by vectors is prevented.

Papaya

Virus-resistant papaya is an exemplary example that demonstrated the promise biotechnology holds. It literally saved an industry that could disappear. A recent survey by USDA suggested that papaya yields increased by 33% in 2000 compared to 1998, which is a direct consequence of using PSRV-resistant plants (USDA-NASS, 2001b).

Squash

Biotechnology-derived virus protection in squash translated to increased number of harvests and increased yield per harvest. Evidence suggests that virus-resistant squash produces greater marketable yields of high quality fruit, particularly in production areas where high virus incidence limits the growing season both in terms of number of plantings and number of harvests per planting (Fuchs et al., 1998; Schultheis and Walters, 1998). However, virus-resistant squash has not reduced insecticide use as chemical applications that control aphids also control white flies and will be made to biotechnology-derived squash also.

Benefits of Commercialised Biotechnology-Derived Crops in the United States

Agricultural biotechnology has been a significant step forward inpest management in the United States. The range of its applications has been extensive and is expanding rapidly. The principal commercialised applications thus far include herbicide tolerance, insect resistance and virus resistance.

Biotechnology-derived crops were first introduced for commercial production in the United States in the mid 1990s. In spite of the dichotomy of opinion regarding biotechnology-derived crops, adoption has been dramatically rapid in the United States since their introduction. The United States is the principal country that planted most of the biotechnology-derived crop acreage (68% of the global) followed by Argentina (22%), Canada (6%) and China (3%) in 2001 (James, 2001). In 2001, biotechnology-derived crops were planted on 88 million acres of US crop acreage, up by 18% compared to 2000 (James, 2001). James (2001) noted that adoption of biotechnology-derived crops has been the highest ever for new agricultural technologies and attributed this to grower satisfaction and significant benefits. Current trends suggest that in the next few years almost all acreage of the major crops grown in North America will be biotechnology-derived.

A conflicting aspect of agricultural biotechnology is the amount of public debate and furore it has generated. Opposing opinions regarding biotechnology-derived crops centre on different perceptions regarding their benefits, environmental and ecological safety, implications on human health and ethics. An understanding of the benefits of agricultural biotechnology for pest management is pivotal to judge the merit of the technology and to resolve the public discussion.

This chapter examines the importance of pest management in crop production and details the commercially available biotechnology-derived traits and their need in the context of available pest management options in conventional crops. The discussion is mainly focused on the actual and potential benefits of this innovation for crops commercialised so far in the United States. Economic advantage to growers is the ultimate key factor, which determines the adoption and success of biotechnology-derived crops. Economic benefits normally result from reduced input costs or increased yields or both.

Importance of Pests in Agriculture

Pest populations have been and will continue to be the major constraints to crop production in the United States. Based on a 1988–1990 estimate, the impact of weeds, insects and pathogens on the production value of eight major crops grown in North America was 11.4, 10.2 and 9.6%, respectively, amounting to a total of $23 billion (Oerke et al., 1994). A recent estimate suggests that impact of weeds alone on US economy exceeds $20 billion annually (Bridges, 1994).

Pest management in crops is a dynamic activity that evolves as new technologies are developed. Growers have relied on a variety of tactics such as manual methods, cultural practices, biological control, quarantine and pest-resistant cultivars to combat pests thus far. Use of chemicals replaced manual and cultural methods in the 1940s, after which crop productivity increased dramatically in the United States.

Weeds are a constant and major challenge to farmers worldwide. About 72% of the pesticides used in the United States are herbicides, 21% are insecticides and 7% are fungicides (Duke, 1998), which emphasises the importance of weeds as crop pests. Control of weeds is critical as they compete with crops for nutrients, water, sunlight and space resulting in significant yield and quality losses. Season-long weed infestations can result in severe yield losses depending on the competing weed species and their density. For example, corn yields were reduced 10% by giant foxtail, 11% by common lambsquarters, 18% by velvetleaf and 22% by common cocklebur at a density of two per foot of row (Beckett et al., 1988). Additionally, weeds increase the cost of agricultural production, reduce land use and human efficiency, and act as hosts for insects and pathogens thereby increasing their control costs. As a result, almost all of the acreage of major crops in the United States is treated with herbicides to avoid yield loss.

Insects infest crop plants for the most basic reasons: to obtain food or protection for overwintering and oviposition or to complete their life cycle. The direct impact of insects result from their feeding on plant parts, which leads to reduction in crop productivity and quality. Losses due to insects each year in the United States were estimated to be 13% or $33 billion (USBC, 1998). The concentrated large acreages of a single crop in successive years (monoculture) have led to a general increase in insect pest populations in the United States. Monocultures lead to unstable agroecosystems due to increased abundance of food supply, decreased competition, low diversity of insect pests and increased ease of locating food supply.

Pathogens that infect plants fall under diverse groups such as viruses, bacteria, fungi, algae, protozoans and nematodes. These pathogens cause harmful physiological and metabolic effects in crop plants thereby resulting in significant yield losses. For example, estimated crop losses due to diseases in the United States are over 10% (El-Zik and Frisbie, 1991). Annual crop losses to all plant pathogens total an approximate $33 billion in the United States (USBC, 1998).

Crop loss estimates due to various pests are often misleading as they represent average loss over a wide area of production. In reality, losses are usually much higher on individual farms. Thus, crop losses definitely justify research to explore new methods such as modern biotechnology to manage pest populations.

The following discussion centres on why growers have adopted biotechnology-derived crops. It outlines the shortcomings in conventional pest management and suggests how biotechnology-derived crops offer solutions to these problems. Finally, the benefits derived from the technology for specific crops are highlighted.

The Criollo genome has uncovered the genetic basis of pathways leading to the most important quality traits of chocolate

The production of high quality chocolate, and the farmers who grow it, will benefit from the recent sequencing and assembly of the chocolate tree genome, as per an international team led by Claire Lanaud of CIRAD, France, with Mark Guiltinan of Penn State, and including researchers from 18 other institutions.

The team sequenced the DNA of a variety of Theobroma cacao, considered to produce the world's finest chocolate. The Maya domesticated this variety of Theobroma cacao, Criollo, about 3,000 years ago in Central America, and it is one of the oldest domesticated tree crops. Today, a number of growers prefer to grow hybrid cacao trees that produce chocolate of lower quality but are more resistant to disease.

"Fine cocoa production is estimated to be less than 5 percent of the world cocoa production because of low productivity and disease susceptibility," said Guiltinan, professor of plant molecular biology.

The scientists report in the current issue of Nature Genetics "consumers have shown an increased interest for high-quality chocolate made with cocoa of good quality and for dark chocolate, containing a higher percentage of cocoa, while also taking into account environmental and ethical criteria for cocoa production".

Currently, most cacao farmers earn about $2 per day, but producers of fine cacao earn more. Increasing the productivity and ease of growing cacao can help to develop a sustainable cacao economy. The trees are now also seen as an environmentally beneficial crop because they grow best under forest shade, allowing for land rehabilitation and enriched biodiversity.

The team's work identified a variety of gene families that may have future impact on improving cacao trees and fruit either by enhancing their attributes or providing protection from fungal diseases and insects that effect cacao trees.

"Our analysis of the Criollo genome has uncovered the genetic basis of pathways leading to the most important quality traits of chocolate -- oil, flavonoid and terpene biosynthesis," said Siela Maximova, associate professor of horticulture, Penn State, and a member of the research team. "It has also led to the discovery of hundreds of genes potentially involved in pathogen resistance, all of which can be used to accelerate the development of elite varieties of cacao in the future".

Because the Criollo trees are self-pollinating, they are generally highly homozygous, possessing two identical forms of each gene, making this particular variety a good choice for accurate genome assembly.

The scientists assembled 84 percent of the genome identifying 28,798 genes that code for proteins. They assigned 88 percent or 23,529 of these protein-coding genes to one of the 10 chromosomes in the Criollo cacao tree. They also looked at microRNAs, short noncoding RNAs that regulate genes, and observed that microRNAs in Criollo are probably major regulators of gene expression.

"Interestingly, only 20 percent of the genome was made up of transposable elements, one of the natural pathways through which genetic sequences change," said Guiltinan "They do this by moving around the chromosomes, changing the order of the genetic material. Smaller amounts of transposons than found in other plant species could lead to slower evolution of the chocolate plant, which was shown to have a relatively simple evolutionary history in terms of genome structure".

Guiltinan and colleagues are interested in specific gene families that could link to specific cocoa qualities or disease resistance. They hope that mapping these gene families will lead to a source of genes directly involved in variations in the plant that are useful for acceleration of plant breeding programs.

The scientists identified two types of disease resistance genes in the Criollo genome. They compared these to previously identified regions on the chromosomes that correlate with disease resistance -- QTLs -- and observed that there was a connection between a number of the resistance genes' QTL locations. The team suggests that a functional genomics approach, one that looks at what the genes do, is needed to confirm potential disease resistant genes in the Criollo genome.

Hidden in the genome the scientists also found genes that code for the production of cocoa butter, a substance highly prized in chocolate making, confectionary, pharmaceuticals and cosmetics. Most cocoa beans are already about 50 percent fat, but these 84 genes control not only the amounts but also quality of the cocoa butter.

Other genes were observed that influence the production of flavonoids, natural antioxidants and terpenoids, hormones, pigments and aromas. Altering the genes for these chemicals might produce chocolate with better flavors, aromas and even healthier chocolate.

Source

With fungi on their side, rice plants grow to be big

Morning dew gathering on rice leaves at a farm. Researchers at a Swiss university said Thursday that they have uncovered a microscopic fungus that is able to increase the speed of rice growth by five times.

By tinkering with a type of fungus that lives in association with plant roots, researchers have found a way to increase the growth of rice by an impressive margin. The so-called mycorrhizal fungi are found in association with nearly all plants in nature, where they deliver essential nutrients—specifically phosphate—to plants in return for sugar. The findings are nevertheless a surprise, according to researchers reporting online on June 10th in Current Biology, because there has been little evidence thus far to suggest that crop plants actually respond to the fungi.

"Global reserves of phosphate are critically low, and because the demand for phosphate goes hand in hand with human population expansion, it is predicted that there will be major shortages in the next few decades," said Ian Sanders of the University of Lausanne in Switzerland. "Unfortunately, most of our important crop plants do not respond strongly, if at all, to inoculation with these fungi. This is especially so for rice, the most globally important food plant. There are no clear reports that rice benefits from inoculation with mycorrhizal fungi."

That is, until now. In fact, the researchers started with a strain of mycorrhizal fungus of the species Glomus intraradices that clearly didn't benefit rice. They then took advantage of the fungus's unusual genetics. A single fungal filament can contain genetically distinct nuclei. Those distinct nuclei can fuse together, mixing genes up in different combinations, and fungal spores can also end up with different complements of genes, the new research shows. As such, the supposedly clonal fungi maintain a degree of genetic variation that had been overlooked.

"It turns out we can very simply manipulate their genetics to produce fungi that induce up to a five-fold growth increase in this globally important food plant," Sanders said.

The genetic changes that the researchers produced in the fungi led to changes in the activity of important genes in the rice, they report. Those affected genes are known to be involved in establishing the mutually beneficial relationship between plant and fungus and in the transport of phosphate at the interface between fungus and plant.

Sanders emphasized that the genetic manipulation the researchers undertook didn't involve any insertion of new genes into the fungal genome. It rather relied on the same biological processes of genetic exchange and segregation that normally take place in the fungus. "What we have done with these fungi is not much different from what plant breeders, and farmers before them, have done to improve crops," he said. "The only difference is that the genetics of these fungi is a little bit more unusual, and no one thought it worth doing."

On a cautionary note, Sanders did emphasize that the plants they studied were grown in a greenhouse in Switzerland under conditions that only mimicked those found in the tropics. "This is clearly not at all the same environment as a rice plant growing in a real paddy field," he said. It remains to be seen whether the same growth benefits will apply in practice.

"However," Sanders said, "our study clearly shows that the potential is there to manipulate the genetics of the fungus to achieve greater crop yields."

More information: Angelard et al.: “Report: Segregation in a Mycorrhizal Fungus Alters Rice Growth and Symbiosis-Specific Gene Transcription.” Publishing in Current Biology 20, 13, July 13, 2010. DOI 10.1016/j.cub.2010.04.043

Provided by Cell Press