Horticultural crops are grown under a wide range of environmental conditions. Horticulturists generally attempt to control as many environmental factors as possible that influence plant growth, such as light, temperature, moisture, and nutrients. More of these factors can be controlled in the greenhouse than in the field. The assumption is that these are the main factors that influence plant growth, but one of the most critical ones, soil, often is overlooked. Even when soilless mixes are used, little thought may be given to controlling more than moisture and nutrients in the medium, although most researchers and growers recognize the need for mixes to have good aeration and drainage, capacity to hold nutrients, and freedom from insects, diseases, and weeds as well as harmful chemicals. The latter viewpoint is largely correct, but overlooks the role played by nonpathogenic soil microorganisms in the growth and development of plants.
A 2-year study was done in Oregon to determine the effects of irrigation method and level of water application on the development of root rot in northern highbush blueberry (Vaccinium corymbosum L. ‘Duke’). Plants were grown on mulched, raised beds and irrigated by overhead sprinklers, microsprays, or drip at 50%, 100%, and 150% of the estimated crop evapotranspiration requirement. Soil at the site was a silty clay loam. By the end of the first season, plants were largest with drip, intermediate-sized with microsprays and smallest with sprinklers; however, this was not the case the next season. By the end of year 2, plants irrigated by drip had less canopy cover, fewer new canes, lower pruning weights, and only half the shoot and root dry weight as plants irrigated by sprinklers or microsprays. Destructive sampling revealed that the field was infested by root rot. Less growth with drip was association with higher levels of infection by the root pathogen, Phytophthora cinnamomi. Phytophthora infection increased with water application, regardless of irrigation method, but averaged 14% with drip and only 7% with sprinklers and microsprays. Roots were also infected by Pythium spp. Pythium infection likewise increased with the total amount of water applied but, unlike P. cinnamomi, was similar among irrigation methods. Overall, drip irrigation maintained higher soil water content near the base of the plants than sprinklers and microsprays, resulting in conditions more favorable to root rot. Sprinklers and microsprays may be better alternatives than drip at sites prone to problems with the disease.
Formation and function of arbuscular mycorrhizae (AM) are affected by levels of fertility in soil or fertilizers applied to soilless container mixes. For AM fungi, phosphorus (P) is the main element influencing colonization of host plant roots. The question addressed in this study was whether inorganic or organic fertilizers were more compatible with the formation and function of AM. Several controlled-release inorganic (CRI) fertilizers were compared with several organic (OR) fertilizers at different rates (½× to 4× the recommended rate) to determine (1) threshold levels of tolerance by the AM fungus Glomus intraradices in relation to root colonization, and (2) growth responses of `Guardsman' bunching onion (Allium cepa) and `Orange Cupido' miniature rose (Rosa spp.) plants grown in a soilless potting mix or sandy loam soil. AM colonization in soil was greatly decreased or totally inhibited by CRI fertilizers with high P content at the 2× rate or greater, whereas colonization was decreased but never eliminated by low-P OR fertilizers at the 3× rate or greater. Shoot growth of onions was similar with or without AM inoculation when fertilized with CRI, but in general was only enhanced by OR fertilizers if inoculated with AM fungi, compared to the noninoculated controls. Shoot and root growth of onions were significantly increased by AM inoculation when OR fertilizers were used at the 1× rate. In contrast, root growth was not increased by the combination of CRI fertilizers and AM fungal inoculation. Inoculation of miniature roses grown in sandy loam amended with 25% peat and perlite and fertilized with all the CRI or OR fertilizers resulted in high AM colonization, but without much AM-induced growth increase except where OR fertilizers or CRI fertilizers with low P were used. In a soilless potting mix, growth of miniature roses was less with OR fertilizers at the rates used than CRI fertilizers, but mycorrhiza formation was greater in the former unless P was low in the latter. These results indicate that release of nutrients from organic fertilizers, as a result of microbial activity, favors AM establishment and function more than most inorganic fertilizers unless P levels of the latter are low.
Coconut fiber dust (coir) is being used as a peat substitute or amendment to potting mixes with varied results. However, its microbial composition and compatibility with beneficial microbes that might be added to growth media in the nursery, such as mycorrhizal fungi, has not been determined. In this study, coir was amended to a peat-based medium (15%, 30%, 45%, and 60% by volume) to determine its effects on growth of several ornamental plants and on the formation and function of the arbuscular mycorrhizal (AM) fungus Glomus intraradices. Mycorrhizae formed as well, and usually better, in all the coir-amended peat treatments as in peat alone. The magnitude of growth enhancement due to mycorrhizae was small for the plants tested in these media compared to that which usually occurs in soil-based media. In this experiment, plant growth responses appeared to be independent of level of mycorrhizal colonization and were plant species dependent. Consistent growth enhancement from mycorrhizae only occurred with marigold (Tagetes patula). With germander (Teucrium fruticans), growth was depressed with mycorrhizal inoculation in the medium composed of 60% coir. Growth of lavender (Lavandula augustifolia) was depressed in all coir-amended media, with or without AM inoculation, compared to the nonamended control. These results confirm previous reports of varied response of plant species to coir, and indicate the lack of any detrimental effects of coir on mycorrhiza formation.
Formation of arbuscular mycorrhizae (AM) has been inhibited in soilless potting mixes that usually contain some proportion of peat moss. The cause of the inhibition has been thought to be high fertilizer P content in the media that suppresses spread of the fungal symbiont in the root tissue. However, there has also been some suggestion that the peats themselves may contribute to the inhibition. That possibility was explored in this study. A sandy-loam soil, in which mycorrhizae consistently enhance plant growth under P-limiting conditions, was amended with six different peats. Onions (Allium cepa 'White Lisbon'), as an indicator host, were grown in the mixes under P-limiting conditions, and were inoculated or not with the AM fungi Glomus deserticola or Gigaspora rosea. Plant growth response to inoculation with AM fungi (AMF) varied with the type of peat and AMF isolate. Inoculated plants generally had the highest root biomass when grown in soil amended with peat. Root colonization by the two fungal symbionts was also affected differently by different peat amendments. Root colonization by Glomus deserticola and Gigaspora rosea was inhibited by at least half of the peat types. However, the types of peat inhibitory to Gigaspora rosea colonization were not the same as those inhibitory to Glomus deserticola colonization. These results indicate that different peat amendments can suppress or enhance mycorrhiza formation on onion roots and resultant growth benefit under P-limiting conditions, depending on the mycorrhizal fungus used.
The establishment and performance of vesicular–arbuscular mycorrhizae (VAM) formed by Glomus fasciculatum (Thaxter) Gerd. & Trappe were studied on geranium (Pelargonium × hortorum L.H. Bailey) and subterranean clover (Trifolium subterraneum L.) in various growth media at 2 P fertility levels. Colonization by G. fasciculatum was not extensive and shoot dry weight and P uptake consequently were not increased by VAM in soilless media such as peat, bark, perlite, and vermiculite. In media containing soil and fertilized at the low P level, roots were colonized extensively by G. fasciculatum, and host shoot growth and P concentrations were increased by VAM. Host growth enhancement by VAM was not observed at the higher P fertility level. Differences in colonization and mycorrhizal response in different fertilized growth media were correlated negatively with the logarithm of the equilibrium solution P concentration. Colonization, growth response, and P uptake by geranium inoculated with G. mosseae (Nic. & Gerd.) Gerd. & Trappe or Acaulospora spinosa Walker & Trappe were affected by growth medium and P fertilizer in the same way as plants inoculated with G. fasciculatum. Peat mosses from different sources varied considerably in their effects on mycorrhiza formation by G. fasciculatum, and on growth response of geranium when the peat was diluted with different amounts of soil. These differences appeared to be related to the equilibrium solution P concentration of the fertilized peats, and not to extractable P of the unfertilized peats. Use of rock phosphate or bonemeal instead of NaH2PO4 as a source of P did not improve the establishment of VAM and host growth response in soil, peat, or vermiculite. Addition of 5–10% Turface, bentonite, silt loam soil, or clay subsoil to peat or vermiculite resulted in increased colonization of host roots and significant mycorrhizal growth response, whereas amendment with liquid sludge inhibited formation of mycorrhizae.
Composted materials with high humic and microbial content, and their water extracts, are increasingly used in the nursery industry as potting mix components or as liquid amendments for the purposes of enhancing plant growth. Common speculation is that such materials either contain beneficial microbes or stimulate those in or added to the medium, such as vesicular-arbuscular mycorrhizal (VAM) fungi, known to have growth-stimulating effects on plants. Experiments were conducted to determine if one such compost enhanced plant growth by stimulating VAM fungi or other growth-enhancing microbes, by simply providing limiting nutrients [phosphorus (P)], or a combination of the two. Highly mycorrhiza-responsive onion (Allium cepa) `White Lisbon' was used to evaluate the interactions of composted grape pomace (CGP), the VAM fungus Glomus intraradices, and preplant soil heat treatment on onion growth under P-limiting conditions. CGP and its water extract stimulated onion growth under P-limiting conditions in the absence of VAM; the extract was more effective than the granular CGP. Growth was enhanced further by addition of G. intraradices, and the extract enhanced its colonization of roots. Heat pretreatment of the soil inconsistently affected growth-enhancement by CGP or its extract. Thus, inoculating plant roots with mycorrhizal fungi in combination with this composted organic amendment or its extract was beneficial. The effect could have been due to the CGP providing a source of P to overcome the P-limiting conditions, and to the mycorrhizal fungus enhancing P uptake by its extraradical hyphae and thereby increasing nutrient-use efficiency.
Phytophthora ramorum survived in potting media infested with sporangia or chlamydospores, allowing the pathogen to remain undetected while disseminated geographically. Chlamydospores or oospores of P. ramorum, Pythium irregulare, Thielaviopsis basicola, and Cylindrocladium scoparium produced in vermiculite culture were used to infest potting media. Infested media in plastic plug flats were treated with aerated steam mixtures from 45 to 70 °C for 30 min. In a second experiment, infested media were fumigated in polyethylene bags with a concentration series of metam sodium ranging from 0.25 to 1.0 mL·L−1. Survival of the pathogens was determined by selective baiting or direct plating the infested media on PARP selective medium. Assays indicated that all pathogens in the infested potting media were killed by aerated steam heat treatments of 50 °C or higher. Metam sodium concentrations of 1.0 mL·L−1 of medium or greater also eradicated all pathogens from the potting medium and soil. These results show that aerated steam treatment or fumigation with metam sodium can effectively sanitize soil-less potting media infested with P. ramorum or other soilborne pathogens, as well as P. ramorum-infested soil beneath infected plant containers. In addition, steam treatments to 70 °C did not melt plastic plug trays.
Inoculation of the various potting media with the mycorrhizal fungus Glomus fasciculatum (Thaxter) Gerd. & Trappe resulted in root colonization of geranium (Pelargonium × hortorum L.H. Bailey) by the endophyte that persisted after transplanting. By flowering time, mycorrhizal transplants grown at 11 or 43 ppm P were more uniform in growth than nonmycorrhizal transplants grown at the same nutrient regimes, had greater leaf areas and leaf weights, had increased root and shoot weights, and had lower foliar Mn concentrations. Mycorrhizae were sufficiently well-established on most seedlings at a minimum of 2 weeks after emergence to persist and continue to develop after transplanting. Pretransplant inoculation with G. fasciculatum increased subsequent geranium growth over that of nonmycorrhizal controls when both were transplanted into soil heavily infested with mycorrhizal fungus inoculum, even though size of mycorrhizal and nonmycorrhizal plants did not differ at the time of transplanting. Posttransplant inoculation with G. fasciculatum did not affect the growth of plants which were already mycorrhizal at transplant, but increased the growth of previously nonmycorrhizal transplants. Pretransplant inoculation in soil, peat, or vermiculite resulted in larger plants than posttransplant inoculation in these media.
When inoculum of ectomycorrhizal fungi was added to the rooting medium, the percentage of rooted cuttings and the root volumes on cuttings of bearberry (Arctostaphylos uva-ursi L. Spreng.) and huckleberry (Vaccinium ovatum Parsh) were significantly greater than those of the uninoculated controls. This enhanced rooting occurred before or in the absence of any mycorrhizal association. In some tests, inoculum of one fungus enhanced rooting of one cultivar of bearberry, but not another, suggesting a specific interaction between the cultivar and the fungus. Of the 13 fungi tested, only Thelephora terrestris Ehrh. ex Fr. formed ectendomycorrhizae in the propagation bed, although several others did so under other conditions.