Low-input management practices for container Ericaceous nursery crops
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.
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.
Impacts and Contributions/Outcomes
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.
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