December 15, 2013
Managing verticillium wilt of maples using organic amendments
Autumn Blaze and sugar maples were used in the verticillium trials.
By Dr. George Lazarovits
A and L Biologicals, Agroecology Research Services Centre

Verticillium dahliae is one of the most unusual plant pathogens in the world. Most plant pathogens can infect only a few species of plants, but  Verticillium dahliae can infect over 400 plant species, including herbaceous annuals such as cotton, eggplant and potato, and perennials such as mint, and woody species such as maple and olive.

Verticillium wilt symptoms of maple include wilting and chlorosis of leaves, die-back of one or more branches, usually from the top, stunting of trees, and eventually, but not always, death.

The fungus produces long-lasting resting structures called microsclerotia (MS). These can persist in soil for over a decade. Reducing the populations of MS in soils is the primary means of disease control and has been achieved using fumigation with chemicals such as chloropicrin and Vapam, in the form of steam or solarization. More recently, organic amendments such as Brassica meals and poultry manures (Goicoechea, 2009) have been used.

The objective of the research by A and L Biologicals was to implement biorational disease management methods to suppress infection by Verticillium and other soilborne fungal pathogens in maple varieties sold in Ontario. Results have shown that incorporation of organic soil amendments such as molasses, poultry manure, meat and bone meal, and mustard meal can significantly enhance plant vigour and growth of maple seedlings.

The researchers established plots in the spring of 2010 where they treated soils with pelletized poultry manure, meat and bone meal, fish emulsion, molasses, and an untreated control. The plots were set out as four replicates in a randomized block design. Half of each plot was covered with a clear plastic sheet to heat the soils using solar radiation (solarization).

Two ft. tall bare root maple seedlings were planted after four weeks, with each plot having ten seedlings per half plot (uncovered vs. plastic). Twice during each summer measurements were collected over three years that included tree height, trunk diameter, and vigour. No significant differences were found between trees in solarized and non-solarized plots. The best performing plants were found in soil treated with molasses where a 30 per cent increase in growth was observed as compared to trees in untreated control plots.

Tree growth was also increased in soils treated with poultry manure and meat and bone meal, but plants growing in soil treated with fish emulsion did not perform as well than those in untreated plots. The growth response in the molasses, poultry manure, and meat and bone meal treated sites were statistically significant and plants were 26-44 per cent taller than those in untreated plots. They also showed a significant increase in girth of 11-40 per cent, and improved overall vigour based on visual appearance.

In the fall of 2010 a site with a previous history of verticillium wilt of maple, received a fall treatment of mustard meal (known to control apple replant disease) and poultry manure, a material that is readily available to growers. A fall application was chosen as it was more suited to growers’ practices, allowing them to plant trees early in the spring. It was expected that any damage done to the resting structures of the Verticillium would be exacerbated by winter damage. Half the site was treated and the other half was used to plant seedlings treated with biological agents that may antagonize Verticillium. Treatments included untreated control, poultry manure at 0.5 per cent v/v, and Brassica seed meal at 1.5 and 3 tons/A.

There were four replicates of four treatments arranged in a randomized design. After application, bags of Verticillium microsclerotia were buried into the soil to determine the efficacy of the amendments for killing MS. Of the inoculum recovered in the spring, the total numbers of viable MS were poultry manure 35, full mustard 6, half mustard 25 and control 55. This indicated a significant reduction in inoculum using the mustard amendment.

Two rows of seven to eight plants per plot of sugar maple and Autumn Blaze maples were planted into plots in the spring. Both trees were planted in the untreated field, but their roots were dipped into a mixture of Pseudomonas bacteria (strains P25, P119, P153), proven antagonists of many soilborne plant pathogens, a Lysobacter species that was found to strongly antagonize the growth of Verticillium on agar (strain 158), a commercial biological control agent (Trichoderma), or just the carrier solution as a control.
Plant heights and vigour were measured as previously. Plants treated with Pseudomonas were the most vigourous whereas, while those treated with Lysobacter were the least vigourous. The mustard seed treatment provided the best growth with Autumn Blaze and again, Pseudomonas the best vigour. Plants growing in soils with the full mustard were 33 per cent taller than in the untreated plots. The tallest sugar maples were found in the poultry manure treated plots, but this was not statistically significant.

Autumn Blaze maples grew significantly better in mustard treated soil although the results were similar to all the organic treatments. The Autumn Blaze trees growing in soil treated with half mustard were about 26 per cent taller than those in untreated plots. Part of this growth effect may be due to increased fertility as there were no visible disease symptoms of Verticillium in any of the plots at either site. The strains of Verticillium that had caused wilt of maple previously in these soils did not appear to be highly virulent to the maple species we planted.

A promising technology for control of verticillium wilt has been described by Goud et al. (2004) and has been termed biological soil disinfestation (BSD). BSD involves incorporating large volumes of organic amendments to soil, wetting the soils to saturation, and then covering them with plastic to create anaerobic conditions. Maples planted in BSD soil treatments showed 85 per cent reductions of soil inoculum levels of V. dahliae relative to the control. The numbers did not increase over four years. Nematode populations, which can exacerbate wilt disease, were also reduced by 95 to 99 per cent. The incidence of infection by V. dahliae of trees was reduced by 80 to 90 per cent in all four years at one location and for the first two years at the other. Most important, the market value of the crop in BSD plots was higher for maple than in the untreated control. These authors suggest that BSD can be an effective, economically profitable, and environmentally friendly control method for tree nurseries (Goud et al. 2004).

In order to identify if the disease was coming in with seedlings, we examined over 450 maple whips produced by growers in Canada and U.S. All were found to be free of Verticillium infections using isolations methods on semi-selective agar media. Surface sterilized sections of roots from such trees, however, were extensively colonized by numerous fungal species, which unidentifiable as to the species level. The research team developed a quantitative molecular method for detection of Verticillium and these tests showed the plants free of Verticillium infection.

Application of organic amendments to soils in the fall provides a simple technology for growers to control crop losses related to soilborne plant disease and to do it in an environmentally safe manner. The benefits of applying such amendments can also be seen in the form of increased productivity such that the growth cycle may be reduced by potentially one year. To this point, test results using root dips of biological control agents such as Trichoderma and Pseudomonas spp. have not yet show any increase in tree growth. However, treatments with Pseudomonas strains, a family of bacteria known to create disease suppressive soil conditions, consistently produced the most vigourous plants by visual appearance. This suggests that they may be increasing overall root health and perhaps it is expected that we will see increased tree quality in subsequent years of growth with this treatment.

The researchers acknowledge the help of A&L Biologicals’ staff and the co-op students from the University of Western Ontario. John Mantel of Connon Nurseries/AVK Holdings provided invaluable guidance during the project and all his efforts toward getting this work done. The research team is grateful to Landscape Ontario Horticultural Trades Association and to AAFC through the Canadian Agricultural Adaptation Program (CAAP) for funding this work.