Two PCR primer pairs, based on the virD2 and ipt genes, detected a wide variety of pathogenic Agrobacterium strains. The endonuclease domain of VirD2 protein, which cleaves transferred DNA (T-DNA) border sequences, is highly conserved; primer oligonucleotides specific for the endonuclease portion of virD2 detected all pathogenic strains of Agrobacterium tested. PCR primers corresponding to conserved sequences in ipt, the T-DNA-borne cytokinin synthesis gene, detected only Agrobacterium tumefaciens and distinguished it from Agrobacterium rhizogenes. The virD2 and ipt primer pairs did not interfere with each other when included in the same PCR amplification, and this permitted simultaneous detection of both genes in a single reaction. One nonpathogenic Agrobacterium radiobacter strain contained virD2 but not ipt; we speculate that this strain arose from a pathogenic progenitor through a deletion in the T-DNA. The virD2 primer pair appears to be universal for all pathogenic Agrobacterium species; used together, the primer sets reported here should allow unambiguous identification of Ti plasmid DNA in bacteria isolated from soil and plants.

Mycorrhizae improve plant nutrient uptake and are known to affect the water relations of plants grown in growth chambers and greenhouses. This paper summarizes a 3‐yr field study that tested the effects of mycorrhizae and water management on the growth and grain yield of maize (Zea mays L.). In each year, two inoculation treatments (inoculated or not with Glomus etunicatum Becker and Gerdemann) and three water‐management treatments (fully irrigated, moderate stress, and severe stress) were applied to fumigated and fertilized Millhopper fine sand (loamy, siliceous, hyperthermic Grossarenic Paleudult). Inoculum was placed in a furrow 10 cm deep at an average rate of 1500 propagules per meter of row. Six to 7 wk after planting, colonization ranged from 0 to 6% of total root length on noninoculated plants and from 10 to 61% on inoculated plants. Twelve to 13 wk after planting, colonization ranged from 2 to 30% on noninoculated plants and from 21 to 56% on inoculated plants. Water stress had little effect on root colonization. By 52 d after planting, one more leaf had appeared and one additional leaf had formed a collar on inoculated plants. Inoculation increased the concentrations of P and Cu in both shoots and grain on all measurement dates. Overall, grain yields (0.306) and total above‐ground biomass yields (0.458 Mg ha‐1 cm‐1 of water) increased linearly with irrigation. A positive response to mycorrhizal inoculation was constant across irrigation levels (0.802 for grain and 1.170 Mg ha−1 for biomass). Therefore, the proportional response of maize to inoculation with G. etunicatum increased with increasing drought stress.

SUMMARYIsolates of Glomus macrocarpum Tul. & Tul. grown in soil cultures from single spores were tested at known numbers of propagules ml"' soil for their efficacy in promoting growth of bell pepper {Capsicum annuum L.) seedlings. Tbe soil cultures, propagated under similar growth conditions, bad from 0-02 to 25 propagules ml ' as measured by the most probable number tecbnique. Seven cultures witb similar levels of inoculum density (a median of 7 propagules ml"') were used undiluted, and at 10"' and at 10"'^ dilutions to test eflficacy of tbe isolates. .•Mso, cultures witb originally dissimilar propagule levels (0-02 to 15 ml"') were diluted to uniform inoculum density levels and tested for efficacy.Large intra-species variation in efficacy occurred in isolates collected from different soils and in those isolated from the same soil. Differences in the growth enhancement ability of isolates were evident from four weeks after seeding until harvest. Tbe best isolates were superior at ail inoculum levels tested but the ranking of isolates differed with the inoculum density.A significant positive correlation was found between initial G. macrocarpum propagules ml"' soil and all plant heigbt and weigbt parameters measured. Growth of mycorrhizal plants was significantly greater than that of non-mycorrhizal plants when more than 05 propagule ml"' was present. With more tban 5 ml"', 70",, of the isolates were effective. A minimum level of 1 propagule ml"' is suggested for infestation of pepper seed beds; greater benefits can be expected from bigher levels up to at least 10 to 15 propagules ml"'.

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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 vesiculararbuscular 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.