Influence of Formononetin and NaCl on Mycorrhizal Colonization and Fusarium Crown and Root Rot of Asparagus

Replanted asparagus fields commonly fail to produce a profitable stand due to alleopathic residues left behind from the previous asparagus crop, elevated densities of pathogenic Fusarium spp., and low densities of vesicular arbuscular mycorrhizae (VAM). Formononetin, a plant isoflavone that stimulates VAM spores to germinate, and sodium chloride (NaCl), a disease-suppressing amendment, were evaluated alone and in combination for their effect on reestablishing asparagus at two locations in abandoned asparagus fields. Greenhouse studies also were conducted with naturally and artificially infested soils. Formononetin was applied as a crown soak or soil drench, and NaCl was applied as a granular treatment. Feeder roots from soil cores sampled from field plots and from greenhouse transplants were assayed for colonization by VAM and for lesions caused by Fusarium oxysporum and F. proliferatum. Formononetin increased the number of VAM vesicles in roots from the field and greenhouse studies and reduced the percent root lesions caused by Fusarium spp. when compared with the nontreated controls. NaCl was more effective than formononetin in reducing the percentage of root lesions in both field and greenhouse experiment when compared with untreated plants but had no effect on VAM colonization. However, there was evidence that NaCl negated the effect of formononentin on VAM colonization. The NaCl treatment increased the May 2001 spear number by 15% and marketable spear weight by 23%. At one site, treatment with formononetin increased mean number of stalks per plant by 29% in 2000 and 14% in 2001. Both formononetin and NaCl improve growth and reduce disease of asparagus in replanted asparagus and may be useful in reestablishing asparagus in abandoned asparagus field.

Survey of vesicular–arbuscular mycorrhizae in lettuce production in relation to management and soil factors

The occurrence of vesicular–arbuscular mycorrhizae (VAM) root colonization and spore number in soil was assessed for 18 fields under intensive lettuce (Lactuca sativa L.) production in California during July and August of 1995. Data on management practices and soil characteristics were compiled for each field, and included a wide range of conditions. The relationship between these factors and the occurrence of VAM in these fields was explored with multivariate statistical analysis. VAM colonization of lettuce tended to decrease with the use of chemical inputs, such as pesticides and high amounts of P and N fertilizers. Addition of soil organic matter amendments, the occurrence of other host crops in the rotation, and soil carbon[ratio ]phosphorus and carbon[ratio ]nitrogen ratios, were positively associated with VAM colonization of lettuce roots. The number of VAM spores in soil was strongly correlated with the number of other host crops in the rotation, the occurrence of weed hosts and sampling date, but was more affected by general soil conditions than by management inputs. Higher total soil N, C and P, as well as CEC, were inversely related to soil spore number. A glasshouse study of the two primary lettuce types sampled in the field showed no significant differences in the extent of root colonization under similar growing conditions. The results of this study are compared with other studies on the effects of management and soil conditions on mycorrhizal occurrence in agriculture.

The Effect of Four Composts on the Establishment of Vesicular–Arbuscular Mycorrhizae in Soilless Media

The beneficial use of vesicular–arbuscular mycorrhizae (VAM) in mineral soils is well-documented, but little is known about the effect of soilless mixes on mycorrhizal colonization of roots. Previous research indicates that mycorrhizal colonization is affected by pH, soluble salts, phosphorus levels, cation exchange capacity, percent organic matter, and some peats. No other research has been published, to our knowledge, on the role of commonly used horticultural composts and mycorrhizal establishment. This study examined four different composts for their effect on VAM establishment using onion roots as an indicator. The composts used in the study were vermicompost, spent mushroom compost, yard waste compost, and processed manure fiber. Plant growth parameters, phosphorus (P) levels and rate of desorption, and microbial populations were analyzed in relation to the percent of VAM colonization of the roots. Significant differences were found in percent VAM colonization between composts. The primary factors influencing VAM colonization were the initial levels of P in the blends and the rate and amount of P released. The experiment raised questions about the balance between mineralized P and organic P in composts and their effect on VAM fungal spore germination.

Local reduction of mycorrhizal arbuscule frequency in enriched soil microsites

Increased nutrient availability reduces vesicular–arbuscular mycorrhizal (VAM) associations with plants, but whether increased nutrients in small volumes of soil affect local VAM colonization is not known. In a field experiment we investigated VAM colonization at different times following fertilization of small soil patches. Soil volumes of ~ 1000 cm3 were treated with a nutrient solution (enriched patch) or distilled water (control patch) on opposite sides of individual plants of the tussock grass Agropyron desertorum and the shrub Artemisia tridentata. Agropyron had significantly lower (p = 0.03) arbuscular infection in the locally enriched patches compared to control patches (32 and 40%, respectively). This reduced arbuscule frequency was apparent at the first sampling (3 days following treatment application) and remained lower in each subsequent sampling (as much as 30% lower than the control patches). Artemisia revealed a similar pattern in arbuscule frequency but was not statistically significant. Our results suggest that a plant can locally reduce VAM development, since arbuscule frequency specifically was locally reduced even though vesicle and overall infection was not. Since mycorrhizal infection does not increase, we conclude that increased plant root proliferation and uptake capacity are likely to be more important for the exploitation of temporary nutrient pulses or patches than is increased mycorrhizal activity. Key words: arbuscule, nutrient exploitation, phosphorus, reduced development, regulation of colonization, soil heterogeneity, vesicular–arbuscular mycorrhizae.

EFFECT OF IRRIGATION AND PRETRANSPLANT INOCULATION OF VESICULAR-ARBUSCULAR MYCORRHIZAL (VAM) FUNGI ON POSTTRANSPLANT GROWTH OF PROSOPIS ALBA GRIEB

Prosopis alba (mesquite) in 27-liter containers, either infected or noninfected with the VAM fungi, Glomus intraradix Schenk & Smith, during the container production phase were transplanted into a simulated landscape and irrigated at regular intervals or nonirrigated (irrigated only once at transplanting time). Mesquite shoot extension (SE), trunk diameter, rooting density (RD), specific root length density (SRLD), and VAM colonization levels were measured at 6 months and 1 year after transplanting. At 6 months, VAM colonization was observed only in the roots of inoculated mesquite, but by 12 months, roots of inoculated and noninoculated mesquite were colonized by VAM fungi. There were higher levels of VAM colonization in roots of irrigated mesquite (23%) in comparison to nonirrigated mesquite (5%). Irrigation promoted SE and VAM inoculation inhibited SE of nonirrigated trees. Trunk diameter was greater for irrigated trees than for nonirrigated trees and was not affected by VAM fungal treatment. At 6 months after transplanting, VAM fungal and irrigation treatments interacted to affect mesquite RD and SRLD. For irrigated mesquite, RD and SRLD were highest for VAM-inoculated mesquite, whereas for nonirrigated trees, RD and SRLD were highest for noninoculated trees. At 12 months after transplanting, mesquite RD and SRLD were higher for irrigated than for nonirrigated trees and were not affected by previous VAM inoculation.

Effects of benomyl, clipping, and competition on growth of prereproductive Lotus corniculatus

Vesicular–arbuscular mycorrhizae, defoliation, and competition can influence survival, growth, and fecundity of plants, but the combined effects of these factors are not well known. I examined how combinations of these factors influence biomass allocation and investment in root nodules by prereproductive Lotus corniculatus and whether the effects were ephemeral. Soil with vesicular–arbuscular mycorrhizal (VAM) fungi was treated with the fungicide benomyl or water and added to trays containing two L. corniculatus or one L. corniculatus and one Brassica napus (a nonmycotrophic species). Leaves of target L. corniculatus were undamaged or clipped five times over 40 days. Plants were harvested 5, 18, or 36 days after last clipping. Interspecific competition was the dominant effect at all harvests: B. napus greatly depressed growth of its neighbor. Benomyl depressed VAM colonization only in the first harvest, and growth reduction associated with depressed colonization diminished over time. Clipping reduced growth most in plants paired with conspecifics, but growth depression was transient. Benomyl and clipping reduced mass of root nodules in the first harvest. Benomyl reduced root mass in nontarget (competitor) L. corniculatus, but plants recovered with time. Neither benomyl nor clipping of the target plant affected B. napus. Interactions were few, indicating that the effects of factors were mostly additive. Key words: VAM fungi, resource allocation, nonmycotrophic competitor.

Occurrence and isolation of vesicular–arbuscular mycorrhizae in cropped field soils of Saskatchewan, Canada

Soil and root samples collected from fields cropped to spring wheat (Triticum aestivum L. cv. Katepwa) and lentil (Lens esculenta L. cv. Eston) at 11 sites across four soil zones of Saskatchewan were analyzed for spore numbers, level of vesicular–arbuscular mycorrhizal (VAM) colonization, and VAM species. The number of VAM spores detected in field soils ranged from 78 to 272 per 100 g soil. Vesicular–arbuscular mycorrhizae colonized wheat and lentil at all the field study sites, but levels of colonization in the two crops varied from site to site and the differences were more pronounced in wheat than in lentil. Generally, lentil both exhibited a higher percentage of VAM colonized roots and contained more arbuscules and vesicles than wheat roots. However, wheat appeared to be colonized by different types of VAM depending on the field sites. Differences in VAM colonization were not related to the moisture and temperature gradient of the four soil zones or soil properties. Seven VAM species were isolated by enriching indigenous VAM mixtures (collected from wheat field soils of six field sites) on maize. The VAM isolated most closely resembled Acaulospora denticulata, Gigaspora decipiens, Glomus clarum, Glomus etunicatum, Glomus fasciculatum, Glomus mosseae, and Glomus versiforme. The species composition of the VAM community varied at the different field sites.Key words: VAM, Acaulospora, Gigaspora, Glomus.

Vesicular-arbuscular mycorrhizae (VAM) improve phosphorus and zinc nutrition and growth of pigeonpea in a Vertisol

In a glasshouse trial, pigeonpea (Cajanus cajan) was grown in a Vertisol from the Darling Downs, Qld. The experimental design included two rates of inoculation with vesicular-arbuscular mycorrhizal (VAM) fungi (nil and inoculated), three rates of phosphorus (P) application and two of zinc (Zn), and inoculation (nil and inoculated) with a recently discovered pathogen of pigeonpea, Phytophthora drechsleri Tucker (Pdr). lnoculation with the pathogen was included in the factorial design to investigate any effect of VAM on root rot. Plants responded to inoculation with VAM fungi, showing that the growth of pigeonpea is highly dependent upon mycorrhizal colonization of its root system. The mycorrhizal plants yielded, on average, 3.3 times the dry weight of the non-mycorrhizal plants. Shoot dry weight and Zn concentration of the shoots were both exponentially related to per cent VAM colonization of the root length ( R2 values of 0.904 and 0.644 respectively) and P concentration was linearly related to VAM colonization (R2 = 0.888). VAM increased P concentration, P uptake, Zn concentration, Zn uptake and P/Zn ratio, indicating enhanced growth through improved P and to a lesser extent Zn nutrition. Zinc fertilizer (15 mg kg-1 soil) without Phytophthora inoculation was fungitoxic to the mycorrhizae, decreasing per cent VAM colonization and depressing plant growth. lnoculation with Pdr did not result in infection and root rot symptoms. However, it did overcome the Zn toxicity, possibly through organic chelation effects, and thereby enhanced VAM colonization. Greatest VAM colonization and best plant nutrition and growth were obtained with the combination of VAM inoculation, Pdr inoculation, Zn (15 mg kg-1) and P (10 mg kg-1). Our results indicate the importance of maintaining adequate levels of VAM fungi in soil through appropriate agricultural practices in order to maximize pigeonpea growth.

Mycorrhizal Species, Root Age, and Position of Mycorrhizal Inoculum Influence Colonization of Cotton, Onion, and Pepper Seedlings

The length of time required for vesicular-arbuscular mycorrhiza (VAM) colonization, the effect of root age, and the position of VAM inoculum with respect to the root system were tested on cotton (Gossypium hirsutum L.), onion (Allium cepa L.), and pepper (Capsicum annuum L.). Colonization of onion by Glomus deserticola began 3 days after inoculation and reached 50% of the total root length after 21 days. Colonization by G. mosseae and G. intraradices began after 12 days and attained 15% and 37%, respectively, after 21 days. In cotton, colonization with G. deserticola and G. intraradices began 12 days following inoculation and increased to 20% and 18%, respectively, after 21 days. Colonization of cotton by G. mosseae was poor. In pepper, colonization with G. deserticola, G. mosseae, and G. intraradices began 3, 6, and 6 days after inoculation and, after 21 days, reached 60%, 13%, and 10%, respectively. In a second experiment, rapid colonization by G. deserticola took place in 3-day-old onion seedlings and increased to 51% 3 days after inoculation. Ten- and 17-day-old seedlings were far less responsive to VAM colonization but became highly infected at 30 days when new roots were produced. In a third experiment, inoculum placement 3 cm below seeds at planting in the field was the most effective for promoting colonization of cotton and onion by VAM. In fumigated field soil, mycorrhizae increased cotton growth an average of 28% when inoculum was applied below seeds compared to one- or two-sided band applications. Even in nonfumigated field soil, inoculum placed 3 cm below the seed and inoculum placed in a band at one side 2 weeks after planting significantly increased cotton growth. In onion, mycorrhizal inoculation improved growth in fumigated soil when it was placed below the seed, but did not stimulate growth in nonfumigated soil.