Nursery growers constantly look for ways to produce high quality and low-input container-grown nursery plants with a minimum impact on the environment. The integration of naturally-occurring mycorrhizal fungi proved to increase plant growth and minimize nutrient runoff. Recently, commercial ericoid mycorrhizal inoculum became available and nursery growers were interested in learning about the feasibility and impact of using various sources of mycorrhizal fungi and fertilizer rates on azalea’s plant growth using two cultivars Delaware Valley White (DVW) and Silver Sword (SS), root colonization and susceptibility to Phytopthora cinimomi infection. In collaboration with two nursery operators, a comparative study was conducted in south Jersey and replicated (without Phytophthora segment) at the Rutgers Fruit Research and Education Center at Cream Ridge, NJ. Our results showed that at all locations, DVW plants grew larger than SS irrespective of substrate growing medium. At the research center, there was a significant increase in above-ground and root plant biomass for DVW when grown in peat-based growing medium. Upon overwintering, azalea plants of both cultivars had greater biomass when grown in bark-based than in peat-based growing medium. Azalea DVW plants had greater shoot tissue N concentration than those of SS irrespective of substrate medium. Before overwintering, mycorrhizal colonization was higher in DVW than in SS plants, however, after overwintering, mycorrhizal colonization was low in all root samples between cultivars and treatments but colonization was four times higher in SS than DVW in the bark-based medium.
At the grower’s site, the overall root biomass was highest in plants grown in bark-based medium and treated with natural mycorrhiza and half rate of fertilizer. In peat-based medium, top biomass of DVW was highest in plants inoculated with natural source of mycorrhizae and received half fertilizer rate, whereas, DVW and SS root biomass was higher in treatments that received lower fertilizer rates than those fertilized with higher rates. While Phytophthora infection did not lower significantly root biomass of SS cultivar, the infection reduced DVW root biomass by 50% or 30% when grown in bark- or peat-based medium, respectively. In any growing medium, non-infected DVW or SS plants had higher N tissue concentration than infected and especially in treatments that received higher rate of fertilizer. In this study, while there was no significant difference in root mycorrhizal colonization among cultivars, treatments, phytophthora infection, or in their interactions in bark-based medium. However, mycorrhizal colonization was almost three-fold higher for DVW than for SS in peat-based medium.
For a sustainable nursery crop production, the addition of natural mycorrhizal fungi at half fertilizer rate will produce larger root biomass and larger above-ground mass when grown in peat-based growing medium. The selection of Phytophthora-tolerant azalea plants such as SS cultivar will provide better biomass and nutrient content than Phytophthora-sensitive DVW cultivar.
We conducted a comparative study that assessed the impact of commercially-available ericoid mycorrhizal inoculum and naturally-occurring inoculum on plant growth and its quality, the combined benefit from using mycorrhizal fungi and fertilizer rates below industry standards, and the tolerance of plants to disease caused by the water mold pathogen Phytophthora cinnamomi. As a result, we anticipated to develop a feasible, cost effective, and environmentally friendly low-input production system for ericaceous plants.
The objectives of this study were to assess: a) top and root biomass, b) plant nutrient content, c) extent of root colonization by mycorrhizal fungi from commercially available and natural sources (both before and after overwintering), d) root and crown rot incidence and severity, e) impact of chlorination on mycorrhizal colonization, and f) the economic feasibility of using the proposed sources in producing high quality plants.
In 2008, an experiment was arranged in a randomized completely block design with four replications at the Rutgers Fruit Research and Education Center, Cream Ridge, New Jersey. Rooted liners of azalea cultivars [Delaware Valley White (DVW), and Silver Sword (SS)] were transplanted into two substrate growing media (peat- and bark-based). There were six treatments, inoculated with either natural or commercial mycorrhzae and fertilized with either full (standard grower’s practice) or half rate fertilizer Nutricote 17-7-8 at time of planting and compared to non-inoculated plants with full and half rate fertilizer. The plants were inoculated with mycorrhizae at time of transplanting. Another set of plants was also planted at the same location and time and monitored to determine the effect of overwintering on ericaceous mycorrhizal root colonization. The same treatments were carried out at two nursery sites located in Bridgeton, south New Jersey. At nursery site 1, the plant cultivars were planted into the nursery’s standard substrate growing media (bark-based) and at site 2 were grown in peat-based medium using same treatments and plant cultivars. These plants were monitored and compared to another set of plants that received Phytophthora cinnamomi inoculation in August 2008. At all sites the plants were irrigated with overhead irrigation (the standard irrigation system used by the nursery growers). At all sites, plant dry top and root biomass were determined at harvesting (October 2008). Root subsamples were collected and assessed for percent mycorrhizal root colonization and percent infection of roots by Phtophthora per plant (at two nursery sites). The data was statistically analyzed using SAS software. Mycorrhizal root colonization in overwintered plants was assessed in root subsamples that collected from plant roots upon harvesting in May 2009.
Our results showed that at the research center, DVW plants grew larger than SS irrespective of substrate growing medium. There was a significant increase in above-ground and root plant biomass for DVW when grown in peat-based growing medium. Azalea DVW plants had greater shoot tissue N concentration than those of SS irrespective of substrate medium, however, plants with full rate of fertilizer with or without mycorrhizae had greater shoot tissue N concentration than those with half rate. Azalea SS plants had slightly higher tissue P concentration and higher tissue K than those in DVW when grown in peat-based. While tissue Cu concentration was higher in SS than in DVW, Fe and Mn were similar in both species. Tissue Na concentration was lower in both plant species when inoculated with either source of mycorrhizal fungi and with half rate of fertilizer. Mycorrhizal colonization was higher in DVW than in SS plants.
Overwintering effect: At the research center, we tested the effect of overwintering on plant biomass and mycorrhizal colonization. Azalea plants of both cultivars had greater biomass when grown in bark-based than in peat-based growing medium. Mycorrhizal colonization was low in all root samples between cultivars and treatments but colonization was four times higher in SS than DVW in the bark-based medium.
At the grower’s site 1, plants were grown in bark-based medium. The DVW azalea plants had larger overall above-ground and root biomass than SS species. The overall root biomass was highest in plants treated with natural mycorrhiza and half rate of fertilizer. Azalea plants infected with P. cinnamomi were lower by 10% in their above-ground biomass than non-infected (Images 1 and 2). Roots of DVW were highly and negatively affected by Phytophthora while those of SS were not significantly different from those non-infected with Phytophthora (Image 3). Our study showed that DVW plants grown in bark-based seemed to be sensitive to P. cinnimomi.
Uninfected plants had 20% higher tissue N concentration than Phytophthora infected ones.
In this study, there was no significant difference in root mycorrhizal colonization among cultivars, treatments, Phytophthora infection, or in their interactions in bark-based medium.
At grower’s site 2- azalea plants were grown in a peat-based growing medium. Above-ground plant biomass of DVW was higher than that of SS. DVW plants inoculated with natural mycorrhizal fungi with half the rate of fertilizer had greater above-ground biomass than those grown in other treatments. There was no significant effect on SS plants. Root biomass in both plant species was higher in treatments that received half fertilizer rate than those with full rate. Phytophthora-infected DVW plants had lower above-ground and root plant dry biomass than non infected ones.
Plant tissue N in DVW was higher than that in SS, and plants with higher fertilizer rate had greater N concentration in their shoot tissue. Uninfected plants had greater N concentration than infected ones. RMCI was almost 3 times higher for DVW than for SS in peat-based medium. Across all plant species, when plants were not infected with Phyophthora, mycorrhizal colonization was highest in treatments that received natural mycorrhizae with half or full fertilizer rate, whereas in the infected ones, mycorrhizal colonization was highest in non-mycorrhizal inoculated plants at half rate of fertilizer.
In this study we were unable to test the impact of chlorination in root mycorrhizal colonization simply because it was not practical for the grower to set up irrigation system specifically to irrigate the plants with or without chlorinated water especially when his irrigation system in his operation changed totally to chlorination by the time we set up the experiment and for the other nursery site, chlorination or any other disinfection system were not installed and running by the time we established the experiment.
The results of this study suggest that the tested azalea plant cultivars vary in their root growth when planted in different substrate growing media and in their sensitivity to P. cinnimomi.
In our study we were able to identify the following: 1) DVW azalea plant cultivar was susceptible to P. cinnomomi and SS was tolerant, 2) the interaction of substrate growing medium (peat-based) and Phytophthora infection could reduce plant top dry weight, especially for DVW, 3) plant biomass of DVW with natural mycorrhizae at half rate of fertilizer provided higher plant biomass and was comparable to those inoculated with commercial mycorrhizal at full fertilizer rate, 4) SS inoculated with any source of mycorrhizae and fertilized at full rate provided significantly higher biomass when compared to other treatments.
Hence, in developing a low-input system for the production of azalea plants in container it will be necessary to select P. cinnimomi-tolerant plant cultivars such as Silver Sword to reduce plant losses to root rot diseases. While mycorrhizal inoculum may not serve as an added benefit for SS against root rot disease, the use of mycorrhizae to grow nursery azalea plants, Delaware Valley White cultivar, was beneficial and provided better growth especially when these plants were exposed to P. cinnimomi and fertilized at a half the grower’s standard fertilizer rate.
Education & Outreach Activities and Participation Summary
We shared the information of this study and its results with two hundred and sixty five people including nursery growers, educators, specialists, researchers, students, and commercial companies. The results of this study have been summarized and presented at national, regional, and local conferences. The results were also documented as published abstracts in the American Society for Horticultural Science (ASHS). We compiled the data and were included in a manuscript to be published in a refereed journal.
1. Zinati, G.M. 2011. Managing Container Media Mixes to Optimize Water and Nutrient Management. Presented at the 2011 Long Island Horticulture Conference, Holiday Inn, Ronkonkoma, NY. Sponsored by Cornell Cooperative Extension, Suffolk County, January 28, 2011.
2. Zinati, G.M. 2010.Differences in nutritional, pest, and environmental damage to rhododendrons. Presented at the American Rhododendron Society, Park Ridge, NJ. Sponsored by the Tappan Zee Chapter, December 4, 2010.
3. Zinati, G.M. 2010. Managing the root environment to optimize water and nutrient management in container-grown ornamental production. Presented at the National Water Conference Symposium 1: Water Issues in the Production of Ornamental Crops in the United States, held at Hilton Head, South Carolina, on February 22, 2010.
4. Zinati, G. M., J. Dighton, and A. Gould J. 2009. Substrate media, fertilizer rate, and mycorrhizal inoculum source affect azalea plant root mycorrhizal colonization and severity of Phytophthora cinnamomi infection. Presented at the 2009 ASHS Annual Conference, St. Louis, MO July 28, 2009. (presentation was chosen for podcast recording at ASHS conference).
5. Gladis Zinati. Management of nursery plants using mycorrhizae. Presented at the South Jersey Landscape Conference and Nursery Growers Meeting, Masso’s Crystal Manor, Glassboro, NJ, December 3, 2008.
6. Gladis Zinati. Future trends in nursery production. Presented at the nursery workshop “Future trends in nursery production: Using natural mycorrhizae a hands-on experience”, June 25, 2008.
Published Refereed Abstracts:
Zinati, G. M., J. Dighton, and A. Gould J. 2009. Substrate media, fertilizer rate, and mycorrhizal inoculum source affect azalea plant root mycorrhizal colonization and severity of Phytophthora cinnamomi infection. HortScience 44(4):1064.
Although, we have not conducted an economic analysis, the cost of mycorrhizae constitutes a minimal expense in comparison to the cost of fertilizer, its transportation, and application.
This study addressed the needs of the New Jersey’s nursery growers in learning whether 1) the type of substrate growing media and source of inoculum (natural vs. commercial) would make a difference in plant biomass of container-grown azalea plants and 2) mycorrhizal inoculation would be beneficial for sustainability of azalea plants when infected with P. cinnimomi under the tested treatments.
The involvement of the nursery growers at each step of the experiment, e.g. selecting the cultivars, fertilizer type, and their input on this study were exemplary of the joint effort between researchers and growers and addressing the SARE Partnership Grant goals. The nursery growers learned about the method of mycorrhizal inoculation from different sources, witnessed the visual differences between treated plants at the nursery sites, and learned how they could manage their production systems by using less fertilizer and Phytophthora-tolerant plants.
Changes in production systems, such as adopting mycorrhizal inoculation and cultivars with less potentiality to P. cinnamomi infection, using reduced rate of fertilizer than what is commonly used may be simple solutions that can be adopted by commercial nursery growers throughout the northeast and across the nation. The commercial ericoid mycorrhizal inoculum is limited. However, through our hands-on workshop that was conducted in 2008 for SARE Research and Education Grant, the nursery growers learned about how to inoculate their plants, and how easy to dedicate a specific area on their nurseries to grow ericaceous plants in ground and use the roots and soil as source of natural inoculum to inoculate plants grown in containers. Recommendations and precautions were also provided to growers to avoid affecting these areas with pesticides, fertilizers, or water runoff that may carry any of these chemicals to ensure viability of the mycorrhizal inoculum.