July 15, 2011
Dr. George Lazarovits, a plant biologist with A&L Biologicals in London, Ont., has submitted his first report on his research into managing verticillium wilt disease of maples and other horticultural plants with organic amendments.

The research project is an initiative of the LO growers sector group. A portion of the funding came from the Farm Innovation Program, a $12-million program that is part of Growing Forward, a federal-provincial-territorial initiative. The research subcommittee of the growers’ sector group worked with the research community to brainstorm and fund research projects that help deal with grower challenges and assist the nursery industry. Dr. Lazarovits’ research project is one that came out of this program. It began in the spring of 2010.

Having just finished one year of research, the study will conclude in the spring of 2012.

Soilborne fungi

Verticillium species are soilborne fungi that cause wilt diseases in over 200 hosts, many of which are of great economic importance. In Holland losses to wilt in tree nurseries are as high as 50 per cent, but specific losses are unknown in Ontario.

The fungus V. dahliae by itself causes serious damage to plants, but in the presence of pathogenic nematodes often kills plants. Such interactions have been found with sugar and Norway maples. V. dahliae produces large numbers of hardy resting structures called microsclerotia. These can survive in soil for a decade. Reducing the population numbers of microsclerotia and those of nematodes is the main aim of most control measures. Microsclerotia, however, can be produced on numerous weed species, making it extremely difficult to eradicate verticillium by using rotation management. Disease symptoms include wilting, chlorosis, stunting, necrosis and vein clearing, and sometimes the presence of brown vascular discolouration in stem tissue cross-sections.

Planting resistant cultivars is the most effective and economical control for wilt. However, few maples are wilt resistant. Broad spectrum soil fumigants are therefore the next best alternative, but these are expensive and come with severe restrictions. As a result, the focus is shifting to managing the disease complex using nonchemical methods.

Application of organic amendments such as poultry manure, fish emulsion, meat and bone meal, molasses, and Brassica seed meals can control the pathogen. A&L Biologicals is working with Landscape Ontario and Canadapt to increase the efficacy of such amendments.

Preliminary results in field trials at two locations have shown that some amendments can effectively reduce inoculum levels. High–nitrogen products, such as poultry manure, can kill microsclerotia by the production of ammonia upon degradation by micro-organisms. As ammonium is released, it causes an increase in soil pH. If pH levels reach higher than 8.0, ammonium starts to be converted into ammonia, which is toxic to verticillium and nematodes. Ammonia rarely forms in soils with high organic matter, as it is rapidly converted to nitrate.

Nitrous acid forms when ammonium is converted to nitrite, which causes soil pH to become acidic. If the soil pH drops below 5.5, nitrite is converted into nitrous acid and this compound is 300 to 500 times more toxic to microsclerotia than ammonia. Nitrous acid is not formed in well-buffered soil. Products such as fish emulsion and molasses kill microsclerotia by the presence of volatile fatty acids (VFA) that include formic, acetic, propionic, butyric, and other similar acids. These materials are all toxic to fungi if the soil pH is lower than 5.5.

Brassica seed meal kills fungi by releasing cyanide gas when it becomes wet. Many other non-chemical techniques can effectively reduce verticillium inoculum, including soil solarization and flooding, but these can take months to accomplish. Biological soil disinfestation (BSD), which combines the incorporation of fresh organic amendments in soil and mulching with airtight plastic, creates anaerobic conditions in the soil and results in pathogen elimination. The method was found to be effective and is advocated for use in high-value crops.

A critical missing component for growers, however, is the ability to detect concentrations of V. dahliae in the soil used for cultivation of susceptible trees. Currently, quantification is done by plating soil on a semi-selective agar medium and using a dissecting microscope to count the colonies of verticillium formed. This method is at best variable and imprecise.

A&L Biologicals has nearly completed the development of a quantitative molecular technique that is based on detection of verticillium and plant pathogenic nematode DNA from extracts of soil and plant tissues. This test is being validated and will be available to growers this coming summer. Once the test is completely effective, researchers will use it to determine the extent of disease introduced with planting material and present in plantations in the field.
Dr. Lazarovits worked at Agriculture and Agrifood Canada until 2010 as a plant pathologist. In 2010 he became director of research at A&L Biologicals in London, Ont.