Diagnosis of light leaf spot (Pyrenopeziza brassicae) on winter oilseed rape (Brassica napus) in the UK

Light leaf spot lesions were generally first observed as light green areas on leaves of UK winter oilseed rape crops in January or February and later became brittle and bleached. Elongated lesions, which were brown with indistinct edges, developed on stems in the spring and summer, when lesions were also observed on flower buds, pedicels and pods. Development of diagnostic white pustules (spore masses of Pyrenopeziza brassicae, which erupt through surfaces of infected tissues) for confirmation of light leaf spot infection on symptomless plants or plants with indistinct or ambiguous symptoms in the autumn, winter or spring was enhanced by incubating plants in polyethylene bags. In experiments with artificially inoculated plants, glasshouse-grown plants exposed in infected crops and plants sampled from crops, white pustules developed at all incubation temperatures from 2^oC to 20^oC on infected leaves of different cultivars. The period of incubation required before the appearance of pustules decreased as the time that had already elapsed since the initial infection increased. The longest periods of incubation were required at the lowest temperatures (2^oC or 5^oC) but leaves senesced and abscised from plants most quickly at the highest temperatures (15^oC or 20^oC), suggesting that the optimal incubation temperature was between 10^oC and 15^oC.     

Effects of temperature, cultivar and isolate on the incubation period of white leaf spot (Mycosphaerella capsellae) on oilseed rape {Brassica napus)

When leaves of oilseed rape (cv. Cobra) were inoculated with conidial suspensions of Mycosphaerella capsellae (white leaf spot) and incubated in controlled environments, the lag period from inoculation to the appearance of the first lesions decreased, and the total number of lesions produced increased, as temperature increased from 5^oC to 20^oC, although differences between 15^oC and 20^oC were small. With incubation period estimated as the time from inoculation until 5%, 50% or 95% of the lesions were produced, there was a linear relationship between l/(incubation period in days) and temperature over the range 5^oC to 20^oC, from which values at intermediate temperatures could be estimated. Summed mean daily temperatures from inoculation to the production of 5% of the lesions were estimated as 115–130 degree-days in the controlled environment experiments at 5^oC to 20^oC. When pods or leaves of plants in oilseed rape crops (cv. Cobra or cv. Libravo) were inoculated with conidial suspensions of M. capsellae on five occasions from January to October, with variable temperatures during the incubation period, degree-days until the first appearance of lesions were in the range 115–230. The numbers of white leaf spot lesions cm^-2 which developed on inoculated leaves differed greatly between nine oilseed rape cultivars, with most on cv. Tapidor and fewest on cv. Libravo, but the incubation period differed little between cultivars. Similarly, the number of lesions which developed differed between four M. capsellae isolates from different regions but the incubation period did not.     

Phytophthora cryptogea root rot of tomato in rockwool nutrient culture:

The effect of root temperature on growth and yield of rockwool-grown tomato plants infected with Phytophthora cryptogea was investigated. Measurements of shoot and root growth were taken at high (25^oC) and low (15^oC) root temperatures during the generative phase of growth. The growth of roots of healthy and P. cryptogea-infected tomato plants in rockwool blocks was higher in plants grown with roots at 25^oC than at 15^oC after 60 days and a similar effect was found in slabs after 98 days. Under sub-optimal conditions for growth the disease became severe when root temperatures were low. Growth of roots was greatest when roots were maintained at a high temperature in combination with an ambient air temperature of c. 15^oC and the response was greater in cv. Counter than cvs Calypso and Marathon. Water-soluble carbohydrates of roots were higher in those produced in blocks than slabs and were reduced by infection compared to healthy plants with roots at 15^oC and 25^oC. Reduced transpiration rates were found 17 days after inoculation in symptomless plants grown at a root temperature of 25^oC. Infection, regardless of the temperature of the roots or cultivar, led to reduced stem growth. The plants grown at 25^oC were taller than those with a root temperature of 15^oC. After 9 wk of harvest, the cumulative fruit yields in infected cvs Counter and Calypso grown at 25^oC were comparable to that in healthy plants grown at either temperature and cumulative fruit numbers followed a similar pattern. High root temperatures led to delayed fruit ripening between weeks 3–10 and a larger number of unripe fruit. The weight of unripe fruit from infected plants grown at 25^oC at the terminal harvest was higher than from healthy plants with roots maintained at 15^oC.     

Properties of an unusual isolate of raspberry ringspot virus from grapevine in Germany and evidence for its possible transmission by Paralongidorus maximus

An isolate of raspberry ringspot nepovirus (RRV-P) commonly found infecting grapevine in localised areas of the German Palatinate, was serologically closely related to, but distinguishable from, the English type strain of this virus (RRV-E) which is transmitted by Longidorus macrosoma. However, unlike RRV-E, RRV-P had a restricted herbaceous host range and produced symptoms reliably in only two hosts, Chenopodium quinoa and Nicotiana occidentalis-accession 37B: these symptoms were a faint systemic vein clearing which, on most occasions in C. quinoa, was transient. In in vitro studies with herbaceous plant sap, RRV-P infectivity was lost after diluting 1/100 -1/500, after storage at 20^oC for 1–3 days and at 4^oC for 45 days: for similar studies with RRV-E, the values were 1/125 000, and more than 15 days at 20^oC and 4^oC, respectively. RRV-P was difficult to purify in quantity and in most preparations seemed to sediment as a single component corresponding to ‘bottom’ component of RRV-E. Purified particles of RRV-P, like those of RRV-E, contained a major polypeptide and two RNA species of M_r 54 000, 2.6 times 10⁶and 1.6 times 10⁶respectively. There was no evidence from RNA preparations from purified virus particles or, from analysis of dsRNA from infected plants, that RRV-P contained a satellite RNA. The incidence of RRV-P in vineyards was not associated with the presence in soils of Longidorus nematodes, but was associated with the distribution in the Palatinate of Paralongidorus maximus. Furthermore, results from an experiment in Germany in a vineyard planted with healthy grapevines in soil fumigated to destroy nematodes, showed spread of RRV-P into these plants from an adjoining source of infected grapevines and soil infested with P. maximus. In laboratory studies, RRV-P was transmitted by P. maximus at a very low level between grapevines (used as the virus source and test plants) but not to, or between, herbaceous hosts.     

Effects of phosphorus and iron fertilizers on the growth of two soybean varieties at two soil temperatures

The influence of FeEDDHA (0, 0.2 and 2 μg Fe g⁻¹ soil) and NaH₂PO₄·H₂O (0 and 120 μg Pg⁻¹ soil) on the growth of two Fe-ineffective soybean (Glycine max L. Merr.) varieties (anoka and T203) on a calcareous soil at two soil temperatures (16 and 24°C) was compared under greenhouse conditions. The two soybean varieties differed in the following respects: (a) T203 accumulated smaller concentrations of Fe in washed tops than Anoka under comparable conditions; (b) T203 was more susceptible to Fe deficiency and its accentuation by high levels of fertilizer P than Anoka; (c) T203 accumulated lower quantities of Mn in tops than Anoka under comparable conditions; (d) T203, but not Anoka, developed Mn deficiency symptoms when treated with P and 2 μg Fe g⁻¹ at 16°C. Fe deficiency was more severe in both varieties at the higher soil temperature due apparently to: (a) greater plant concentration of P in tops at 24°C; and/or (b) an increased rate of plant growth and greater dilution of Fe in young tissue at 24°C. Foliar P concentration was increased much more than foliar Fe concentration by an increase in soil temperature. Severely Fe deficient T203 plants grown without FeEDDHA at 24°C accumulated less foliar Mn than their FeEDDHA counterparts. Comparisons of Fe effectiveness of various soybean cultivars based on relative responses to FeEDDHA can be influenced by differential effects on Mn nutrition.     

The effect of temperature on viability of imbibed weed seeds

Imbibed seed of 10 common arable weeds were placed in trays in initially moist soil and, after imbibing for 2h, heated in ovens/incubators set to 31^oC, 42^oC, 56^oC, 75^oC or 100^oC for 0.5, 1, 2, 4, 8 or 16 days or at 102^oC, 155^oC, 204^oC or 262^oC for 0.5, 1, 2, 5, 7.5 or 10 min. After heating, seeds were incubated for 28 days at 10/20^oC or 20/30^oC on a 12 h dark/light regime, depending on species, and germination recorded. At the lower temperatures, germination of all species was prevented by temperatures of 75^oC or higher for periods of 0.5 days or more. Germination was lower after treatment at 56^oC than at 31^oC or 42^oC for all species except Rumex obtusifolius. The maximum temperature required to prevent germination varied among species and was of greater importance than the duration of heating. Germination was variable with duration of heating. At the higher temperatures, there was very little germination of any species after heating at 204^oC for 7.5 min or 262^oC for 5 min or more. Seeds were greatly buffered from the air temperature by 3 mm of soil, throughout the shorter duration of heating. The average temperature of the soil, over the 10 min heating required to prevent over 90% germination, varied among species and ranged from 48^oC for Avena fatua to 65^oC for R. obtusifolius. This work implies that composting systems maintained at 65^oC are unlikely to provide an efficient method of weed control. Recommendations for improvement of the laboratory technique are suggested.     

Stemphylium botryosum f. lactucae- its developmentjn vifro and its pathogenicity to lettuce leaves

Stemphylium botryosum f. lactucae, incitant of a leaf-spot disease of stored lettuce, was found to be relatively restricted in its host range. Cross-inoculations with spore suspension of this fungus failed to induce symptoms in any of the host plants tested, except carrot. Among isolates of S. botryosum from various hosts, only the isolate from carrot induced slight symptoms on lettuce. While mycelial growth of the lettuce isolate was confined to the range 13–37 ^oC spores germinated at more extreme temperatures. The optimum temperature for germination and for radial growth on PDA was found to be between 25 and 30 ^oC. Wet spores were quickly inactivated at 50 ^oC, whereas more than 40 % of dry spores withstood a 24 h exposure to that temperature. Only the outer leaves of lettuce responded readily to inoculation with a spore suspension, the required incubation period being 3 days at 25 ^oC. Symptoms developed less readily on bruised leaves. Relative humidity approaching saturation was necessary for prompt and typical infection, notably during the 24 h following inoculation. Short dry periods (60 % r.h.) interposed at a later stage, while somewhat inhibitory, did not prevent infection.     

Effects of variation in ear temperature on growth and yield of spring wheat

Wheat plants were kept in a growth room at 15 ^oC from 7 days after an-thesis until maturity and their ears were warmed to 20 or 25 ^oC for various periods. Continuous warming initially increased the growth rate of the ear and decreased that of the stem, but hastened senescence of the ear and decreased final grain yield by decreasing dry weight per grain. Warming the ears increased the movement of ¹⁴C and nitrogen to them from the leaves and stem during the early stages of grain growth but decreased it later. Warming the ears for only the first 10 or 20 days also hastened ear senescence; grain yield decreased progressively with increase in duration of the warm period. All effects were greater at 25 than at 20 ^oC. The effects of changing ear temperature from 15 to 20 ^oC were independent of the temperature of the rest of the plant. Altering the humidity of the air around the ears by 4–7 mb at constant temperature had no effect on ear growth or senescence. Warming all of the plant except the ears from 15 to 20 ^oC increased ear growth slightly during early grain growth and decreased it later, irrespective of ear temperature: stem dry weight, leaf area and net photosynthetic rate of flag leaves and green stems were decreased and dark respiration rate of stems was increased.     

Acclimation of photosynthesis, H2O2 content and antioxidants in maize (Zea mays) grown at sub-optimal temperatures

Maize plants were grown at 14, 18 and 20 °C until the fourth leaf had emerged. Leaves from plants grown at 14 and 18 °C had less chlorophyll than those grown at 20 °C. Maximal extractable ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) activity was decreased at 14 °C compared with 20 °C, but the activation state was highest at 14 °C. Growth at 14 °C increased the abundance (but not the number) of Rubisco breakdown products. Phosphoenolpyruvate carboxylase (PEPC) activity was decreased at 14 °C compared with 20 °C but no chilling-dependent effects on the abundance of the PEPC protein were observed. Maximal extractable NADP-malate dehydrogenase activity increased at 14 °C compared with 20 °C whereas the glutathione pool was similar in leaves from plants grown at both temperatures. Foliar ascorbate and hydrogen peroxide were increased at 14 °C compared with 20 °C. The foliar hydrogen peroxide content was independent of irradiance at both growth temperatures. Plants grown at 14 °C had decreased rates of CO₂ fixation together with decreased quantum efficiencies of photosystem (PS) II in the light, although there was no photo-inhibition. Growth at 14 °C decreased the abundance of the D1 protein of PSII and the PSI psaB gene product but the psaA gene product was largely unaffected by growth at low temperatures. The relationships between the photosystems and the co-ordinate regulation of electron transport and CO₂ assimilation were maintained in plants grown at 14 °C.     

Thermal inactivation of cherry leaf roll virus in tissue cultures of Nicotiana rustica raised from seeds and meristem-tips

Nicotiana rustica tissue cultures derived from seeds or embryos infected with cherry leaf roll virus (CLRV), remained infected after culture at 22 ^oC. No infectivity was found in cultures held at 32 ^oC for 5 days but it was readily detected after such cultures were transferred to 25 ^oC for 8 days. Virus was permanently eradicated from most plants after 20 days incubation at 32 ^oC and from all plants after 7 days incubation at 40 ^oC. Partially purified preparations of CLRV lost infectivity after 9–12 days at 22^oC, 5 days at 32^oC and 3 days at 40^oC.     

Properties of an unusual isolate of raspberry ringspot virus from grapevine in Germany and evidence for its possible transmission by Paralongidorus maximus

An isolate of raspberry ringspot nepovirus (RRV-P) commonly found infecting grapevine in localised areas of the German Palatinate, was serologically closely related to, but distinguishable from, the English type strain of this virus (RRV-E) which is transmitted by Longidorus macrosoma. However, unlike RRV-E, RRV-P had a restricted herbaceous host range and produced symptoms reliably in only two hosts, Chenopodium quinoa and Nicotiana occidentalis-accession 37B: these symptoms were a faint systemic vein clearing which, on most occasions in C. quinoa, was transient. In in vitro studies with herbaceous plant sap, RRV-P infectivity was lost after diluting 1/100-1/500, after storage at 20^oC for 1–3 days and at 4^oC for 45 days: for similar studies with RRV-E, the values were 1/125 000, and more than 15 days at 20^oC and 4^oC, respectively. RRV-P was difficult to purify in quantity and in most preparations seemed to sediment as a single component corresponding to ‘bottom’ component of RRV-E. Purified particles of RRV-P, like those of RRV-E, contained a major polypeptide and two RNA species of Mx 54 000, 2.6 × 106 and 1.6 × 106 respectively. There was no evidence from RNA preparations from purified virus particles or, from analysis of dsRNA from infected plants, that RRV-P contained a satellite RNA. The incidence of RRV-P in vineyards was not associated with the presence in soils of Longidorus nematodes, but was associated with the distribution in the Palatinate of Paralongidorus maximus. Furthermore, results from an experiment in Germany in a vineyard planted with healthy grapevines in soil fumigated to destroy nematodes, showed spread of RRV-P into these plants from an adjoining source of infected grapevines and soil infested with P. maximus. In laboratory studies, RRV-P was transmitted by P. maximus at a very low level between grapevines (used as the virus source and test plants) but not to, or between, herbaceous hosts.     

The effect of elevated temperatures and their duration on the incidence of hollow heart in pea seeds

Plants of Pisum sativum cv. Pania were exposed to high temperature (35 ^oC day/25 ^oC night) at three stages in development. These were: (i) at process crop maturity, when the seed moisture content (m.c.) was 80%, i.e. about 10 days prior to pod wrinkle, (ii) at a seed m.c. of 70 to 80%, about the onset of pod wrinkle, and (iii) when the seed m.c. was 50 to 60%, 10 days after the onset of pod wrinkle. At each stage of development the high temperature treatment was imposed for 2,4,6, 8 or 10 days. Plants exposed to high temperature at seed m.c. of 70 to 80% produced seed with the highest incidence of hollow heart and at all stages of development the incidence increased with the length of exposure to the high temperature. Where plants at the development stage of pod wrinkle (seed m.c. 70–80%) were exposed to a range of temperature regimes for 5 days, over 20% of the seeds had hollow heart when the mean day/night temperature was 25 ^oC. Above a mean temperature of 25 ^oC, the percentage of affected seed increased with increasing day or night temperature. Over 80% of the seeds had hollow heart symptoms after 5 days exposure to a daily mean temperature of 32-5 ^oC. The severity of symptom development was not related to the proportion of seeds with the disorder and, under laboratory conditions, seed germination was not affected.     

Ethylene as a factor limiting the legume-Rhizobium symbiosis in tube culture

Growth and nitrogen fixation by Trifolium subterraneum plants at 15 and 10 ^oC root temperatures is much less in test tubes plugged with cotton wool than in open pots of sand. Removing the ethylene produced by plants growing in cotton wool plugged flasks by adsorption onto activated charcoal, or by aerating with sterile air, increased nitrogenase activity and growth over the first 25 days, to levels approaching that of plants grown in open pots of sand.     

Effect of high soil temperatures on nodulation of cowpea, Vigna unguiculata

Cowpeas, inoculated with one of five effective strains of Rhizobium isolated from African soils, were grown at root temperatures of 30 ^oC continuously or at 36, 38, 40, 42 and 44 ^oC for 5 h/day and returned to glasshouse ambient for the intermediate period. Growth was best at 30 and 36 ^oC; above 40 ^oC growth was poor and no nodules formed. At 40 ^oC two strains failed to nodulate. Symbiotic performance was not dependent only on nodule production as nodule efficiency varied inversely with temperature. The number of nodules formed by strain R5000 after exposure of inoculated seed or seedlings to 40,42 or 44 ^oC for 5 h/day on each of 3 or 6 days depended on the age of plant and the duration of exposure to stress. When exposed to 42 or 44 ^oC during the first 3 days after sowing nodulation was reduced from 18 to 1–3 nodules/plant even after a further 40 days growth at ambient (30 day, 20 ^oC night). Nodulation was unaffected when 10–15-day-old seedlings were exposed to the same conditions. Numbers of strain R5000 on seed declined rapidly following three daily exposures of 5 h at 39 and 42 ^oC; at 45 ^oC less than three bacteria survived on each seed. Other rhizobia of the cowpea group varied greatly in their toleration of high temperatures, some survived well at 45 ^oC whereas others behaved like R5000.     

Effects of variation in temperature and light intensity at different times on growth and yield of spring wheat

Spring wheat grown in pots outdoors was transferred to growth rooms for various periods to study the effect of increasing the temperature from 14-4 to 20-3 ^oC (Expt 1) or from day/night values of 15-0/15-2 to 20-0/15-2 ^oC (Expt 2) and of increasing the amount of visible radiation in a 16 h day from 424 to 792 J cm^-2 (Expt 1) or 374 to 740 J cm^-2 (Expt 2). There were no interactions between temperature and radiation. In Expt 1 neither the increase in temperature nor extra radiation, applied for 14 days immediately after the appearance of double ridges on the stem apex, or 14 days later, increased grain yield at maturity. Warmth early, but not late, increased dry weight, leaf area and the number of floret primordia immediately after treatment, but these effects had disappeared by anthesis, 30 days later. Dry weight but not leaf area was increased by extra radiation but the effects had disappeared 2 wk after treatment. An increase in temperature imposed for 16 days starting 5 or 21 days after anthesis (Expt 2) increased dry weight of the ear and decreased that of the rest of the plant immediately after treatment, and decreased leaf area at all times. When plants from the two temperatures were put together in the same conditions, ear growth of plants that had been in the warm was slower than that of plants from the cold treatment, so that the difference in ear weight observed after 16 days of treatment reversed and grain yield was decreased by warmth applied in either period; the component of yield decreased by warmth was grain size. Additional radiation in either post-anthesis period increased dry weight of all parts of the plant and had negligible effects on leaf area. Final grain yield was increased by c. 15% because the individual grains were larger. Early treatment also increased grain number slightly. The effects of treatment during the two post-anthesis periods were similar in size, and additive.     

Growth and development of soyabean cv. TK5 as affected by tropical daylengths, day/night temperatures and nitrogen nutrition

Two experiments were done in Saxcil growth cabinets in order to investigate the effects of climatic factors and nitrogen nutrition on the growth, reproductive development and seed yield of soyabean cv. TK5. In the first, plants were grown to maturity in eight environments comprising all combinations of two short daylengths (11 h 40 min and 13 h 20 min), two day (27 and 33^oC) and two night (19 and 24^oC) temperatures. In the second, day temperature was kept at 33^oC but the night temperature was varied (19 and 24^oC) as was the mineral nitrogen supply (20 and 197 ppm N) to plants which were either inoculated or not with an effective single strain of Rhizobium. Taller, more branched, later flowering plants were produced in the longer daylength but seed yield was hardly affected because the components of yield did not all respond similarly. In the higher day temperature treatments seed yield per plant was reduced by half because all yield components were adversely affected - pods per plant by 34 %, mean seed dry weight by 24 % and seeds per pod just slightly. There was a marked effect of the higher night temperature which promoted early vegetative growth, induced early flowering and although the number of pods per plant was, overall, reduced by 48 %, seed yield per plant was little affected as mean seed dry weight was increased by 37 % and the number of seeds per pod was also increased slightly. Prior to flowering, nodulated plants obtained about two thirds of their total nitrogen requirement via direct uptake and one third through the symbiotic system. Vegetative dry weight and plant nitrogen content were increased by the higher mineral nitrogen level and, although height was slightly diminished, more branches were produced. Seed yield, however, was only slightly increased. These experiments have shown that night temperature is an environmental factor of major importance for the growth of this soyabean cultivar. They have provided, also, a more rational basis for interpreting seasonal variations in growth and seed yield of soyabean in the tropics where, clearly, day and night temperature effects can override those of daylength and nitrogen nutrition.     

The infection of apples by Phytophthora syringae

Contamination with infected soil has led to high wastage of apples in storage due to rotting by Phytophthora syringae. At 3-3 ^oC lesions formed 3–4 wk after inoculation with zoospores; the percentage infection fell if the suspensions dried after 48 h at this temperature and after 22 h at 15 ^oC. Infected soil rotted fruit only if kept moist; at 3-3 ^oC a 3-day period of wetness resulted in 37-5% rotting after 8 wk. Fruit dipped in soil slurry remained wet in some parts of a 4361(12 bushel) bin for at least 3 wk. There was a 10-fold increase in rotting by contact between sound and rotting fruit after 11 wk at 3-3 ^oC. Captan gave effective protection against rotting derived from zoospores or infected soil; it had no eradicant action.     

Effects of species of VA-mycorrhizal fungi on growth and mineral uptake of sorghum at different temperatures

Sorghum [Sorghum bicolor (L.) Moench] plants were grown in growth chambers at 20, 25 and 30°C in a low P Typic Argiudoll (3.65 µg P g^–1 soil, pH 8.3) inoculated with Glomus fasciculatum, Glomus intraradices, and Glomus macrocarpum to determine effects of vesicular-arbuscular mycorrhizal fungi (VAMF) species on plant growth and mineral nutrient uptake. Sorghum root colonization by VAMF and plant responses to Glomus species were temperature dependent. G. macrocarpum colonized sorghum roots best and enhanced plant growth and mineral uptake considerably more than the other VAMF species, especially at 30°C. G. fasciculatum enhanced shoot growth at 20 and 25°C, and mineral uptake only at 20°C. G. intraradices depressed shoot growth and mineral uptake at 30°C. G. macrocarpum enhanced shoot P, K, and Zn at all temperatures, and Fe at 25 and 30°C above that which could be accounted for by increased biomass. Sorghum plant growth responses to colonization by VAMF species may need to be evaluated at different temperatures to optimize beneficial effects.     

Effects of cold shock on the susceptibility of white lupins to fungal diseases

Experiments in controlled environments tested interactions between freezing soil (a compost-vermiculite mixture) and below-ground infection of white lupins with each of three pathogenic fungi (Botrytis cinerea, Fusarium avenaceum or Pleiochaeta setosa) on plant disease and death. Whilst soil freezing (up to 4 days at – 1^oC) caused slight necrosis and increased the severity of disease symptoms, incidence of plant death was increased only after inoculation, before freezing, of the lower hypocotyl of the youngest plants (soil frozen at less than 17 days old) with P. setosa. It is concluded that the contribution of below-ground infection by pathogenic fungi to overwinter losses in autumn-sown white lupin crops is exacerbated to a negligible extent by soil freezing, the main primary cause of such losses.     

Effects of vesicular-arbuscular mycorrhizal inoculation at different soil P availabilities on growth and nutrient uptake of in vitro propagated coffee (Coffea arabica L.) plants

 In a pot experiment, the growth and the nutrient status of in vitro propagated coffee (Coffea arabica L.) microcuttings were investigated for 5 months following vesicular-arbuscular mycorrhizal (VAM) inoculation with either Acaulospora melleae or Glomus clarum at four soil P availabilities. Control plants remained P-deficient even at the highest soil P availability while mycorrhizal plants were P-sufficient at all soil P availabilities. Growth of control plants was only improved at the highest soil P availability. In P-deficient soil, neither of the two VAM species improved plant growth. Plant growth increased by 50% following inoculation with either A. melleae or G. clarum when P availability went from deficient to low. No further plant growth improvement was induced by either VAM species at intermediate and high soil P levels. Nevertheless, growth of plants inoculated with G. clarum was still significantly greater than that of non-mycorrhizal plants at the highest soil P availability. Root colonization by G. clarum increased with increasing soil P availability while root colonization by A. mellea decreased with soil P level increasing above low P availability. Soil P availability also affected Zn nutrition through its influence on VAM symbiosis. With increasing soil P availability, foliar Zn status increased with G. clarum or decreased with A. mellea in parallel to root colonization by VAM. This study demonstrates the beneficial effects of VAM inoculation on in vitro propagated Arabica coffee microcuttings, as shown previously for seedlings. This study also demonstrates differences in tolerance to soil P availability between VAM species, most likely resulting from their differing abilities to enhance coffee foliar P status. Accepted: 14 November 1996