Jong Hyun Ham
Disease is a major limiting factor for stable and profitable yields of many crops, especially in regions with mild and short winter seasons, like Louisiana, due to higher survival rates of pathogens from overwintering. Plant diseases, like human and animal diseases, are caused by various types of pathogenic microorganisms, including viruses, bacteria, fungi, oomycetes and nematodes. Chemical control is usually a practical and effective way to manage crop diseases caused by fungi, oomycetes and nematodes. Even though chemical control is the most common way to protect crops from many diseases, consecutive usage of similar types of chemical products often results in the occurrence of resistant pathogens, which substantially compromises the efficacy of the chemical agents.
For diseases caused by bacterial pathogens, relatively fewer numbers of chemical products for disease management are available in the market compared to those for the diseases caused by eukaryotic pathogens like fungi, oomycetes and nematodes. Antibiotics used for medical purposes are commonly effective on plant pathogenic bacteria, but usage of antibiotics for agricultural purposes is highly restricted because of the risk of antibiotic-resistance traits, which can be gained by bacteria in the environment through selection pressure and subsequently transferred to clinical pathogens. Copper-based chemical products used as bactericides often show limited efficacy to bacterial diseases as well as phytotoxic effects on certain crops. In the case of viral diseases, there is no chemical available to directly control viral pathogens like human and animal viral diseases.
Because of the risks and limitations of the currently available chemical products, it is imperative to develop new tools and strategies for sustainable disease management. Several attractive alternative disease management options have been commercially available or under development to be released in the market in the near future. These include biological control agents (such as microorganisms antagonistic or pathogenic to pathogens), bacteriophages (viruses that kill bacterial cells), antimicrobial peptides (small peptide compounds produced by various organisms), nanomaterials, and plant defense inducers. However, each alternative disease management option still has its own limitations, including high costs, insufficient stability and activity, and risks to human health and the environment.
Crop production in Louisiana is heavily dependent on chemical products for disease management and, thus, has been facing the problems of chemical management practices and the needs of alternative disease management options addressed above. Especially, challenges are arising in the production of rice, one of the most economically important crops in Louisiana, along with sugarcane and soybeans, because of the current and impending problems in the management of two major rice diseases: bacterial panicle blight and sheath blight. Bacterial panicle blight caused by the plant pathogenic bacteria Burkholderia glumae and Burkholderia gladioli can hardly be managed by existing chemical control options because copper-based products show limited efficacy and phytotoxocity to rice, and antibiotics effective to the pathogens (such as oxolinic acid) are not registered in the U.S. Sheath blight caused by the fungal pathogen Rhizoctonia solani had been effectively managed by the strobilurin-type fungicides, such as Quadris, but fungicide-resistant pathogen isolates have recently been found in numerous rice fields of Louisiana, suggesting the impending crisis of disease management for this chronic rice disease problem.
LSU AgCenter plant pathologists have been working on development of alternative strategies to manage rice diseases, especially focusing on bacterial panicle blight and sheath blight. Various types of materials have been tested for their disease management potential, and some of them have shown promising results as alternative disease management measures on their own or in combination with other materials.
Three types of biological materials have been studied in AgCenter laboratories: antagonistic bacteria, avirulent pathogens and growth-promoting bacteria. Antagonistic bacteria are nonpathogenic bacteria that can suppress the growth of pathogens through production of one or more antimicrobial materials. More than 100 bacterial strains isolated from various parts of rice plants and field soils have shown good antagonistic activities against fungal and bacterial pathogens in the laboratory condition (Figures 1A and 1B), and several of them have exhibited significantly high levels of disease suppression activities for bacterial panicle blight and sheath blight in field conditions as well. In addition, scores of avirulent strains of Burkholderia glumae (the major pathogen of bacterial panicle blight) have been identified in AgCenter laboratories. They lost the ability to produce virulence factors through natural mutation events and, thus, do not cause disease in rice even at a high population level. Some of these avirulent strains showed great antagonistic activities against the sheath blight pathogen Rhizoctonia solani and suppressed the development of sheath blight in the field. Finally, bacteria isolated from the rice root area (rhizosphere) are being tested through seed treatment for their beneficial influences on rice growth and defense. Though use of biological agents is an attractive option for sustainable disease management, there are still several problems to overcome in commercialization of this option, including limited shelf life and potential risk to human and animal health.
Application of nontoxic natural chemicals is an ideal option for disease management if the chosen material is effective, safe and economical. AgCenter plant pathologists have tested a variety of natural chemicals known to have biological activities, such as anti-oxidation and elicitation of plant defense responses, and a couple of them have shown a great efficacy for the suppression of bacterial panicle blight in field tests over multiple years. These materials are promising candidate agents to be used for the management of bacterial panicle blight by themselves or in combination with a bactericide, such as Kocide 3000, as a supplementary component to boost the limited efficacy of the bactericide. Current research activities are focused on this aspect, aiming to develop a more effective product for the management of bacterial panicle blight and other crop diseases caused by bacterial pathogens.
Nanomaterials are generally defined as the materials in a size range between 1 and 100 nanometers (one nanometer is a billionth of a meter). Nanomaterials are applied to a variety of products related to medicine, cosmetics and food engineering. Based on previous reports about the innate antimicrobial activities exhibited by certain nanomaterials, AgCenter plant pathologists have tested various nanomaterials in terms of the antibacterial activity against the bacterial blight pathogen Burkholderia glumae and found that silver nanoparticles and zinc nanoparticles have strong antibacterial activities against the pathogen in the laboratory condition (Figure 2). Because the pathogen is known to be seed-borne, AgCenter plant pathologists are testing the effect of seed treatment of these materials to prevent the development of the disease in the field.
Disease-resistant rice lines
Even though growing disease-resistant varieties is the most economical and environment-friendly way to manage crop diseases, it is generally arduous to develop a crop variety having durable disease resistance. In case of rice, there are few commercial rice varieties showing satisfactory levels of disease resistance to bacterial panicle blight and sheath blight. In an effort to develop commercially valuable disease-resistant rice varieties based on better understanding of rice genetics, AgCenter plant pathologists have been performing genetic and genomic studies of rice focusing on disease resistance traits for bacterial panicle blight and sheath blight, as well as screened rice germplasm showing higher levels of resistance to both diseases. The rice germplasm for screening has been generated from various procedures, including conventional crossings, anther culture and mutagenesis. So far, several lines of rice germplasm have exhibited promising levels of disease resistance to both diseases (See photo at right). They will be an excellent source of disease resistance for the rice breeding programs in Louisiana.
Effective and sustainable disease management cannot be achieved through a single approach. Integration of multiple approaches influencing all the three elements of a disease (host, pathogen and environment) is an ideal disease management strategy, which minimizes economic and environmental risks. However, many of the current disease management practices are heavily dependent on chemical control methods only, such as application of fungicides or bactericides/antibiotics, which increases the risk of resistant pathogens and often encounters diseases for which effective chemical agents are not available. LSU AgCenter research achievements and current efforts will lead to the development of innovative strategies for sustainable disease management of the chronic rice diseases and, further, many other fastidious crop diseases caused by fungal and bacterial pathogens.
Jong Hyun Ham is an associate professor in the
Department of Plant Pathology and Crop Physiology.
Jong Hyun Ham, associate professor in the Department of Plant Pathology and Crop Physiology, displays a tray of young rice plants in his laboratory. He conducts research to find new ways of treating crop diseases in Louisiana. Photo by Olivia McClure
A rice line screened in LSU AgCenter laboratories that exhibits superior disease resistance to both bacterial panicle blight and sheath blight (the three rows of rice in the middle between the two rows of purple rice).
Figure 1A. Antagonistic bacteria exhibiting growth inhibition activities against Burkholderia glumae (the bacterial pathogen causing bacterial panicle blight of rice).
Figure 1B. Antagonistic bacteria exhibiting growth inhibition activities against Rhizoctonia solani (the fungal pathogen causing sheath blight of rice, right).
Figure 2. Bactericidal activities of silver-nanoparticle (Ag-NP) and zinc-nanoparticle (Zn-NP) to the rice pathogenic bacterium Burkholderia glumae. Bacterial cells treated with 0.1% Ag-NP or Zn-NP could not survive in contrast to those without any treatment or treated with 0.1% ascorbic acid.