J. A. Menge

Effect of salinity on mycorrhizal onion and tomato in soil with and without additional phosphate

SummaryVesicular-arbuscular mycorrhizal fungi (VAM) are known to increase plant growth in saline soils. Previous studies, however, have not distinguished whether this growth response is due to enhanced P uptake or a direct mechanism of increased plant salt tolerance by VAM. In a glasshouse experiment onions (Allium cepa L.) were grown in sterilized, low-P sandy loam soil amended with 0, 0.8, 1.6 mmol P kg−1 soil with and without mycorrhizal inoculum. Pots were irrigated with saline waters having conductivities of 1.0, 2.8, 4.3, and 5.9 dS m−1. Onion colonized withGlomus deserticola (Trappe, Bloss, and Menge) increased growth from 394% to 100% over non-inoculated control plants when soil P was low (≤ 0.2 mmol kg−1 NaHCO3-extractable P) at soil saturation extract salinities from 1.1 dS m−1 to 8.8 dS m−1. When 0.8 and 1.6 mM P was added no dry weight differences due to VAM were observed, however, K and P concentrations were higher in VAM plants in saline treatments.Glomus fasciculatum (Gerdeman and Trappe) andGlomus mosseae (Nicol. and Gerd.) isolates increased growth of VAM tomato 44% to 193% in non-sterilized, saline soil (10 dS m−1 saturation extract) despite having little effect on growth in less saline conditions when soil P was low. Higher tomato water potentials, along with improved K nutrition by VAM in onion, indicate mechanisms other than increased P nutrition may be important for VAM plants growing under saline stress. These effects appear to be secondary to the effects of VAM on P uptake.

IMPROVED GROWTH OF TOMATO IN SALINIZED SOIL BY VESICULAR-ARBUSCULAR MYCORRHIZAL FUNGI COLLECTED FROM SALINE SOILS

Vesicular-arbuscular (VA) mycorrhizal fungi were collected and identified from saline locations throughout southern and central California and Nevada. From this collection, 38 soil samples containing VA mycorrhizal fungi from 22 plant species were used to inoculate tomato seedlings and evaluate their growth under artificial salinization. Six samples significantly improved growth of tomato in salinized soil when compared to a salinized, nonmycorrhizal control. Growth with 14 samples in salinized soil was as good as growth of a nonmycorrhizal control in nonsalinized soil. Glomusfasciculatum consistently provided the largest growth response. Negative correlations were found between the amount of mycorrhizal colonization on tomato roots in the greenhouse and electrical conductivity, Na concentration, and osmotic potential of the saline soil samples from the field. The amount of mycorrhizal colonization on hosts in the field from which the soil samples were collected was not correlated to growth response or mycorrhizal colonization of tomato in the greenhouse.

Influence of simazine on formation of vesicular-arbuscular mycorrhizae inChenopodium quinona Willd

SummaryLow levels of vesicular-arbuscular mycorrhiza formation were found in a normally nonmycorrhizal species,Chenopodium quinona, when sprayed with sublethal doses of simazine. Application of simazine also increased exudation of sugars and amino acids from roots. It is suggested that this increase in root exudation is responsible for the unusual formation of mycorrhizae in this species.

Vesicular—Arbuscular Mycorrhizal Fungi Associated with Citrus in Florida and California and Notes on Their Distribution and Ecology

Mycorrhizal fungi were associated with citrus roots or soils in 78 of 79 orchards and nurseries in California and in 64 of 66 citrus orchards and nurseries in Florida. Glomus fasciculatus, G. macrocarpus, G. etunicatus, G. constrictus, and Sclerocystis sinuosa were present in both states. In addition, G. microcarpus and G. monosporus were found in California. Gigaspora margarita and G. mosseae were associated with citrus in Florida but not in California. Glomus fasciculatus was consistently associated with young trees (0-30 yr), whereas G. constrictus was consistently associated with older trees (30-70 yr). Total numbers of mycorrhizal chlamydospores associated with citrus in California were negatively correlated with soil P and organic matter, but were positively correlated with soil Na and pH. Glomus macrocarpus was more frequently found in California citrus soils of high P content than was G. microcarpus, and G. monosporus appeared to inhabit soils of higher B, Ca + Mg, or salinity than several other mycorrhizal fungi. In California, chlamydospores of both G. fasciculatus and G. constrictus were most numerous from November to May and least numerous from June to September.

methods of fosetyl-Al application and phosphonate levels in avocado tissue needed to controls tem canker caused by Phytophthora citricola

The efficacy of several methods of fosetyl-Al application to control avocado stem canker disease, caused by Phytophthora citricola, was evaluated under greenhouse conditions. Fosetyl-Al was applied to the canker area as paint alone (0.4 g a.i. fosetyl-Al + 1.0 ml water), combined with Tree Seal (0.4 g a.i. fosetyl-Al + 0.5 g Tree Seal + 0.5 ml water), or applied alone followed by Tree Seal on either scraped bark or bark cut in a fish-scale-like pattern (30 cm along the stem). The use of fosetyl-Al as a soil drench (3.2 g a.i. fosetyl-Al per liter) was also evaluated. The most effective method was either using the fosetyl-Al:Tree Seal:water formulation on heavily scraped areas of the stem or applying fosetyl-Al alone on the bark cut in a fish-scale-like pattern. Applying fosetyl-Al as a soil drench was also effective in controlling stem canker disease, but to a lesser degree than the paint application method. Phosphonate, the anionic metabolite of fosetyl-Al in plants, was quantified in the bark, leaves, and roots of treated avocado plants by high-performance liquid chromatography following the different application methods of fosetyl-Al. Application of the fosetyl-Al:Tree Seal:water formulation on heavily scraped stem areas resulted in the highest level of phosphonate residue in the canker area and was the most efficacious in controlling the stem canker pathogen. Phosphonate residue in the plant inhibited infection by P. citricola for about 6 months after its application. There was a strong negative correlation (r = -0.978) between the phosphonate level in the stem bark and the size of the stem canker caused by P citricola.

Interaction among mycorrhizae, soil solarization, metalaxyl, and plants in the field

Field trials were conducted in 1988 and 1989 to increase growth response, yield, and vesiculararbuscular mycorrhizal (VAM) fugi colonization of cotton (Gossypium hirsutum), onion (Allium cepa), and pepper (Capsicum annuum) in nonfumigated soil. Treatments of fumigated non nonfumigated soil plots included soil solarization, soil solarization plus tarp coverage for 2 wk after planting, and metalaxyl application. Plants from each treatment were inoculated or not inoculated with VAM fungi (...)

VA-mycorrhizal fungi and soil characteristics in avocado (Persea americana Mill.) orchard soils.

Soils from avocado (Persea americana Mill.) orchards in Israel (IS) and California (CA), both sites with a Mediterranean climate, were sampled and analyzed for the species and quantities of vesicular-arbuscular mycorrhizal fungus (VAMF) spores in them, and for soil physical and chemical characteristics.Numbers of spores were similar in soil from IS and CA but the dominant VAMF species were very different. In IS the most common fungi were Sclerocystis sinuosa and Glomus macrocarpum. In CA, Gl. constrictum was present in every orchard examined and Gl. fasciculatum was nearly as widespread. Acaulospora spp. and other Glomus spp. also were found, including A. elegans which has never before been reported from CA.The differences in VAMF populations and species constituents found on two continents but in areas with similar climates and soil types may be due to host or edaphic factors. Different avocado rootstocks are used in the two countries and lower pH and higher soil fertility levels were present in CA soils.The total VAMF spore populations in each orchard was about 275 per 100 mL soil. The population level was not correlated with any of the soil physical or chemical characteristics examined nor with avocado cultivar or age. In IS no fungus spores were found in three orchards; available P, Ca, Mg and Cu levels were high in these soils.

Urban and agricultural wastes for use as mulches on avocado and citrus and for delivery of microbial biocontrol agents

SummaryUrban and agricultural waste products generally available to avocado and citrus growers in southern California were analyzed for their suitability for use as bioenhanced mulches on citrus and avocado. Of the mulches tested only yard waste (consisting of wood chips, grass and leaves), rice hulls and rice hulls-and-paper were not harmful to any growth parameter of citrus or avocado and were also adequate substrates for the growth of three biocontrol agents: Trichoderma harzianum, Gliocladium virens, and Pseudomonas fluorescens. Several mulches such as milled peanut hulls, milled almond hulls, chicken manure, a horse/cow manure mixture, cow manure and alfalfa hay were poor substrates for growth of the biocontrol agents and were damaging to at least one growth parameter of avocado and citrus. These mulches released toxic amounts of ammonia upon degradation, some in excess of 1000 µg NH3−1 dry wt. The percentage of healthy citrus roots, percentage of healthy avocado roots and growth of T. harzianum and ...

Effect of Pythium ultimum and metalaxyl treatments on root length and mycorrhizal colonization of cotton, onion and pepper.

(...)Five weeks after VAM fungus inoculation, 39-42% of roots were colonized in fumigated soil, compared to 21-26% in nonfumigated soil. VAM colonization of roots increased to 64-71% following treatment with the fungicide metalaxyl. Root lengths and VAM colonization of the three crops were reduced significantly in fumigated soil following infestation with P.u. and were similar to those in nonfumigated soil. Metalaxyl did not affect root length or VAM colonization in fumigated soil. P.u., Fusarium solani and Rhizoctonia solani were isolated from the roots of cotton, onion and pepper grown in nonfumigated soil(...)

Biological control of Phytophthora root rot of avocato with microorganisms grown in organic mulches

Organic mulches colonized with microbial biocontrol agents, termed bioenhanced mulches, were tested for their ability to reduce Phytophthora root rot of avocado (Persea americana Mill.). Benomyl-resistant mutants of Gliocladium virens (KA 230-1) and Trichoderma harzianum (KA 159.2) isolated from suppressive soils and selected as efficient antagonists of P. cinnamomi were evaluated for their ability to colonize different mulches under controlled laboratory conditions. Sudangrass and a coarse yardwaste were found to be better substrates than a fine yardwaste, woodwaste or rice hulls for biocontrol agents propagules production. The most suitable conditions for colonization were an optimum temperature of 24°C, a moisture content of 20% for sudangrass and 30% for the coarse yardwaste, and a continuous light exposure during a 15-day incubation period. In the greenhouse, fresh sudangrass and a coarse yardwaste colonized with G. virens and used as a surface mulch proved to be the best combination for reducing the population of P. cinnamomi in 4-liter pots containing artificially-infested soil. Healthy avocado roots made up 31-37% of the roots in the G. virens-mulch combinations compared to 0% healthy in infested controls after two months.