1a. Objectives (from AD-416)
1) Determine if there is value in adding arbuscular mycorrhizal (AM) fungi inoculum, particularly at planting. 2) Determine if pre-plant fumigation impacts the extent and nature of AM fungal populations in the soil and is this of consequence? 3) Characterize the AM fungi populations present on field grown nursery stock vs. potted plants at the time of planting and the resulting tree performance.
1b. Approach (from AD-416)
Nonpareil almond trees on Nemaguard rootstock will be planted in Spring 2008 at USDA Parlier. Plot strips will be fumigated in the fall of 2007 and non-fumigated plot strips will be used as comparison. An assessment of the extent and nature of AM fungi populations present in the soil/residual roots after fumigation but before planting. At time of planting and before any inoculation, there will be an assessment of the extent and nature of AM present on tree roots. Tree performance data to be collected will include: (1 Trunk circumference: initial and final yearly. (2 Annual pruning weights; Nutrient status-characterize nutritional deficiencies if and when symptoms arise. 3. At end of trial: Whole tree top weight, trunk diameter, etc. Annual assessments will be made of the extent and nature of AM fungal populations on tree roots.
3. Progress Report
Soil borne arbuscular mycorrhizal (AM) fungus forms a symbiotic relationship with most plants. The AM symbiosis improves plant phosphorus, nitrogen and mineral nutrition. Evidence also suggests the symbiosis provides protection of the plant against pathogens and improves plant water relations. The status of AM fungal populations in almond orchards is not well understood. The purpose of this study is to determine if specific agronomic practices, such as pre-plant fumigation and inoculation with AM fungus, have an impact on AM fungal populations and subsequent tree performance. Almond trees planted for this project are now in their third year of growth in fumigated and non-fumigated soils. Trees bloomed for a first time during this growth season and bloom strength was evaluated at full bloom. Blooming progressed similarly throughout the various treatments, but bloom strength differed. Blooming was more intense among trees planted in fumigated plots, regardless of whether trees were bare root/potted or with/without AM treatment. To date in the third year, trunk caliper measurements have been taken twice (752 and 832 days after planting on 1 March 2010 and 20 May 2010, respectively). While no trees have died in the test, several trees in non-fumigated plots appear weak and unhealthy. Five samples were collected from fumigated almond trees and the other from non-fumigated. Roots and surrounding soil were added to a potting mixture seeded with Sudangrass for detecting and isolating AM fungi. Spores were extracted from soils by wet sieving. Sudangrass roots cut in 1-cm segments were stained with trypan blue to detect AM colonization. Spores and stained roots were viewed under a Zeiss stero-microscope. DNA was extracted from spores and colonized roots using PowerSoil DNA isolation kit. Primers from AM fungal ribosomal genes were used for generating PCR fragments. TOPO TA cloning and DNA sequencing were also employed to aid in AM fungal identification. Several hundred clones are being sequenced to determine AM fungal species. Roots of Sudangrass from trap cultures were analyzed for AM fungal colonization in fumigated and non-fumigated soils. The data indicate that fumigation did reduce residual soil AM fungal population. The ADODR monitored this project through site visits, emails and phone calls. The goal of the specific cooperative agreement is to develop practical applications of arbuscular mycorrhizal fungi to reduce aflatoxin in almond, which contributes directly to Objective 2 of the in-house project.