Interaction between zinc nutritional status of cereals and Rhizoctonia root rot severity

An inverse correlation between plant Zn concentration and the severity of Rhizoctonia root rot, described in an earlier paper, was examined in two experiments in a growth chamber. In the first experiment, wheat (Triticum aestivum cv Songlen) was planted in a Zn deficient soil with and without added Zn, and combined factorially with different inoculum densities of Rhizoctonia solani anastomosis group 8. When Zn was added, the percentage of seminal roots infected with R. solani was significantly lower compared to the treatments without added Zn, showing that low Zn potentiated the disease. A subsequent factorial experiment of four inoculum densities and six Zn levels, (0, 0.01, 0.04, 0.1, 0.4 and 6.0 mg Zn kg^–1 soil) was conducted to investigate the Zn effect in more detail. Disease severity was markedly decreased by the higher Zn applications; the disease score dropped sharply between treatments of Zn_0.04 and Zn_0.1, a difference which was reflected in the plant yield response to Zn. For both experiments the Zn concentrations in shoots were significantly different only among Zn treatments, not among the inoculum treatments. This indicated that inoculum density or disease severity did not reduce Zn concentration in the plant. Thus, disease did not exaggerate Zn deficiency, but rather, Zn sufficiency suppressed disease severity. A potentiating link between Zn nutrition and disease severity is thereby established, although this type of experiment did not indicate the mechanism of the Zn effect.     

Radiometric assessment of tillage and seed treatment effect on soybean root rot caused by Fusarium spp. in central Minnesota

Soybean root rot, caused primarily by Fusarium solani f. sp. phaseoli in a complex with F. oxysporum and Rhizoctonia solani, has become an increasing problem for soybeans, dry beans, and other rotation crops in central Minnesota due to soil conditions associated with reduced tillage. This study was conducted, in two field sites in central Minnesota located near Staples and Verndale, to develop methods for nondestructive assessment of root rot severity using plant radiometric properties. Soybean canopy reflectance was measured with a hand-held multi-spectral radiometer. Prior to the radiometer measurements, attempts were made to create differing root rot situations with moldboard or chisel tillage, and with or without a biological seed treatment. Root rot severity was estimated using a visual disease severity scale. Colony-forming units (CFU) were determined to estimate soil populations of pathogenic F. solani and F. oxysporum. Results from the Verndale site consistently showed significant treatment effects in the measured canopy radiometric parameters, and in the visual disease rating and yield (significant for seed treatment). Values of a simple ratio vegetation index from this site exhibited negative relationships with disease rating and F. oxysporum CFU, and a positive linear relationship with yield. Treatment effects were generally not significant at the Staples site because of low initial F. oxysporum populations. The results indicate that remote sensing is potentially a rapid, nondestructive means for assessment of root rot diseases in soybean.     

Zinc fertilisation increases tolerance to Rhizoctonia solani (AG 8) in Medicago truncatula

The effect of Zn fertilisation on tolerance of Medicago truncatula to infection by the root-rotting pathogen Rhizoctonia solani (AG 8) was studied in a field survey and in two experiments in controlled conditions. From the field survey, the concentration of Zn in the shoots of medics was found to be inversely related to the severity of disease on the root. Overall, the addition of Zn to Zn-deficient soil in controlled environment experiments resulted in reduced yield loss in the presence of R. solani, a reduction in disease score and no change in the concentration of nutrients in the shoots. However, under Zn deficiency, increasing levels of added R. solani resulted in significant yield loss, an increase in disease score and a reduction in concentration of Zn in the roots. This occurred despite a decrease in the number of infection sites caused by the fungus on the root and a lower amount of R. solani DNA extracted in medics deficient in Zn compared with plants supplied with Zn. While plants supplied with Zn were able to maintain a stable concentration of Zn in the shoots, the concentration of Zn in the roots also declined with increasing levels of R. solani. In conclusion, Zn application does not directly inhibit infection by R. solani, nor reduce its pathogenicity, but it does strongly increase root growth. The net result is that Zn-sufficient plants are more tolerant to the effects of root pruning by the fungus than Zn-deficient plants.     

The role of cuticle and epidermal cell wall in resistance of rapeseed and mustard to Rhizoctonia solani

A study was conducted to determine whether the cuticles in two genera of the family Cruciferae are effective barriers to infection by Rhizoctonia solani, and whether differences in cuticle and epidermal cell wall thickness and morphology of epicuticular wax exist between resistant and susceptible cultivars. As Canola/rapeseed (Brassica napus) and mustard (Sinapis alba) plants develop from 1 to 3 weeks of age, they become increasingly resistant to R. solani AG2-1 seedling root rot. Seven-day-old seedlings of S. alba cultivars are invariably more resistant than B. napus cultivars. Brassica napus cultivars do not show an obvious cuticle layer at 1 week but at 3 weeks the presence of a cuticle is seen through autofluorescence with a concomitant increase in resistance to R. solani. Removal of the cuticle from 3-week-old hypocotyls by chloroform treatment results in a decrease in cuticular autofluorescence and a significant increase in disease severity in both resistant and susceptible cultivars. Three-week-old plants of S. alba have a much lower percent disease rating and a significantly (p=0.05) thicker cuticle layer than similar-age plants of B. napus. The results suggest that the cuticle plays an important role in the resistance of S. alba and older plants of B. napus to infection by R. solani.     

Effect of mycorrhiza and biofertilisers on reducing the incidence of Fusarium root and pod rot diseases of peanut

Pathogenicity tests of twenty-six fungal isolates were tested on peanut plants (Giza 5 cv.) and the results revealed that, Fusarium oxysporum isolate (No. I) followed by F. solani (No. II) then F. moniliforme (No III) significantly caused highest incidence of root rot disease. Also, F. moniliforme (No III) followed by F. solani (No II) then F. oxysporum (No I) gave the highest incidence of pod rot disease. The effectiveness of vescular arbuscular-mycorrhiza (VAM) at different application rates on the incidence of root rot, pod rot diseases and plant growth parameters of peanut was studied. All soil treatments with each rate of VAM significantly reduced root and pod rot diseases compared with control (rate 0%). The best reduction in the severity of both diseases with VAM was found at the rate of 3%. Application of rhizobacterin, microbin and cerialin biofertilisers at the different concentrations decreased the severity of both root rot and pod rot severity diseases compared with non-treated seeds. The greatest reduction in both diseases was achieved at a concentration of 8/100?g seeds. The highest number of pods and fresh weight (g) was achieved in seed supplemented with each biofertiliser at concentration of 8/100?g seed.     

Density-dependent mortality in Pinus sylvestris caused by the snow blight pathogen Phacidium infestans

Summary Relationships between disease incidence and the density of host plant populations were investigated in the Pinus sylvestris-Phacidium infestans host-fungal pathogen association, in which the season of death of plants killed up to 3 years previously could be accurately determined. Significant (P<0.05), positive density-dependent relationships between the proportion of plants dying in the winters of 1987–1988, 1988–1989 or 1989–1990 and the original stand density were detected in 12 of 26 comparisons. Of the remaining comparisons, all but three had positive regression coefficients for the same association. Plants killed up to 2 years previously contributed to inoculum production. The use of standing dead as a predictor in the analyses showed that the proportion of plants dying in the winters of 1988–1989 or 1989–1990 was generally better correlated with standing dead in the previous summer than with the density of the original population. Significant (P<0.05), positive density-dependent associations were also found between the proportion of living plants in 1990 infected with P. infestans and the number of standing dead plants in all nine comparisons. In contrast, only four of the nine associations between these proportions of infected plants and population density were significant. The strength of the density-dependent relationships varied substantially within and between sites. Much of this variation appears to be due to differences in the stage of development of the epidemics occurring at different sites.     

Interactions Between Rhizoctonia Root Rot and the Foliar Common Bean Diseases Anthracnose and Rust

The effects of co‐inoculation of Rhizoctonia solani and Colletotrichum lindemuthianum or Uromyces appendiculatus at different inoculum levels were studied on the disease dynamics and on the growth of bean plants under greenhouse conditions. Bean seeds were sown in R. solani‐infested soil. Additional experiments in which seedlings were transplanted to infested soil were also carried out. Conidial suspensions of C. lindemuthianum or uredospores of U. appendiculatus were inoculated onto leaves at plant developmental stages V2 and V3, respectively. Interactions between root rot and the aerial diseases were observed depending on the inoculum levels and on the timing of R. solani inoculation. Anthracnose severity tended to be higher on R. solani‐infected plants. Conversely, R. solani infection significantly reduced diameter of pustules and rust severity. When seedlings were transplanted to soil infested with low levels of R. solani, root rot severity and density of R. solani in the soil were magnified at high levels of C. lindemuthianum or U. appendiculatus. In these experiments, a synergistic interaction between root rot and anthracnose was observed to affect the plant dry weight. Antagonistic effects on the plant dry weight were found for the combination root rot/rust only when seeds were sown in infested soil.     

Integration of soil solarization with chemical, biological and cultural control for the management of soilborne diseases of vegetables

The long-term effectiveness of soil solarization integrated with (integration of pest management [IPM]) a biological control agent (Trichoderma virens), chemical fungicide (pentachloronitrobenzene [PCNB]), organic amendment (chicken litter) or physical method (black agriplastic mulch) to reduce southern blight (Sclerotium rolfsii) and southern root-knot diseases (Meloidogyne incognita) were evaluated on vegetable production. Results showed that the long-term effectiveness of IPM plus soil solarization reduced soilborne diseases of vegetables more than two years following the termination of solarization. These disease management strategies in 1991 and 1992, following soil solarization in 1990, reduced the numbers of sclerotia in the soil, and the number of plants killed by southern blight and root-knot of tomatoes, compared to nonsolarized bare soil treatment. The integration of a reduced dosage level of PCNB or T. virens in field plots, reduced southern blight of tomatoes by 100% and 71%, respectively, in solarized soil, compared to nonsolarized bare soil two years following soil solarization. PCNB effectively controlled southern blight in nonsolarized bare soil both years. All solarized treatments, except PCNB plus solarized soil increased tomato yields compared to nonsolarized bare soil plots. In the second study (1992) following soil solarization in 1991, the effectiveness of solarized bare soil, and nonsolarized bare soil mulched with black agriplastic film, with or without Reemay spunbounded polyester row cover, were effective in reducing root-knot of tomatoes as indicated by the root-knot gall index. Following a one year fallow period in 1994 three years following soil solarization, the root-knot gall index for severity of tomato roots grown in solarized bare soil, nonsolarized bare soil, black agriplastic mulched bare nonsolarized soil and black agriplastic mulched solarized bare soil, were 1.0, 3.0, 3.0 and 2.0, respectively, on a 0–5 scale, where 0=0% and 5=100% root-knot galled. In the third study 1992 and 1993, different dosage levels of chicken litter were used to amend soil artificially infested with sclerotia of S. rolfsii at different depths following solarization, decreased the number of viable sclerotia by 85–100%. All solarized treatments and nonsolarized bare soil amended with 18.8 MT/ha of chicken litter, were effective in controlling southern root-knot damage, and postharvest storage root rots of sweetpotato storage roots (Fusarium root rot [Fusarium solani] and Java black rot [Diplodia tubericola]). Our study showed that all soil solarization treatments, and soils amended with chicken litter, stimulated a shift in the soil microbial population dynamics. Rhizobacteria of Bacillus spp. and fluorescent pseudomonads increased significantly in the rhizosphere, rhizoplane, and interior root tissues of tomatoes and sweetpoatoes, grown in solarized soil compared to nonsolarized soil. These microorganisms may have contributed to the increased growth response of vegetables and some were probably suppressive to soilborne diseases     

Efficiency of different application methods of biocontrol agents and biocides in control of Fusarium root rot on some citrus rootstocks

Abstract

The efficiency of two biocontrol agents (Trichoderma harzianum NB and Bacillus subtilis NB) and two commercial biocides (Plant Guard and Rhizo-N) in controlling Fusarium root rot disease on some citrus rootstocks was evaluated under artificially infested soil in green house.

Fusrium root rot on citrus rootstocks seedlings i.e. sour orange (SO), volkamer lime (VL), rangpur lime (RP) and cleopatra mandarin (CL) was successfully controlled by dipping the root system of such seedlings in water suspensions of each biological treatment i.e. Trichoderma harzianum (spore suspension 5×10⁶ spore/ml), Bacillus subtilis (cell suspension 8×10⁷ cell/ml), Plant Guard (3 g/l) and Rhizo-N (4 g/l), then transplanted into artificially infested soil with Fusarium solani and drenched with enough water suspension of such biological treatments. Plant Guard (3 g/l) and Rhizo-N (4 g/l) were highly effective treatments in decreasing infection and severity of the disease, Fusarium density in rhizosphere soil and colonization of Fusarium solani in the roots of all tested seedlings.

Meanwhile, root dipping or soil drenching with the same treatments individually gave the least effect in reducing root rot incidenceon all tested rootstocks compared with application of the two methods together.

It should be noted that using biocontrol agents and commercial biocides could be successfully used in controlling root rot pathogens on citrus in commercial greenhouses or under field conditions before transplanting in new reclaimed lands in the desert.     

Arbuscular mycorrhiza reduces susceptibility of tomato to Alternaria solani

Mycorrhiza frequently leads to the control of root pathogens, but appears to have the opposite effect on leaf pathogens. In this study, we studied mycorrhizal effects on the development of early blight in tomato (Solanum lycopersicum) caused by the necrotrophic fungus Alternaria solani. Alternaria-induced necrosis and chlorosis of all leaves were studied in mycorrhizal and non-mycorrhizal plants over time course and at different soil P levels. Mycorrhizal tomato plants had significantly less A. solani symptoms than non-mycorrhizal plants, but neither plant growth nor phosphate uptake was enhanced by mycorrhizas. An increased P supply had no effect on disease severity in non-mycorrhizal plants, but led to a higher disease severity in mycorrhizal plants. This was parallel to a P-supply-induced reduction in mycorrhiza formation. The protective effect of mycorrhizas towards development of A. solani has some parallels to induced systemic resistance, mediated by rhizobacteria: both biocontrol agents are root-associated organisms and both are effective against necrotrophic pathogens. The possible mechanisms involved are discussed.     

Integrated control of root rot and wilt disease of faba bean by soil amendment with suppressive compost in combination with seed coating with an antagonistic yeast

For this study, 21 isolates of fungi belonging to Rhizoctonia and Fusarium genera were isolated from the diseased faba bean plants, obtained from the different localities in Assiut governorate, showing root rot and wilt symptoms. The isolates proved to be pathogenic on Masr 1 faba bean cultivar under greenhouse conditions. F. oxysporum isolates caused wilt disease; however, the isolates of R. solani and other Fusarium species caused root rot. The virulence of isolates on the tested faba bean cultivar was different. The highly pathogenic isolates of these fungi were employed in this study. The effect of soil amendment with Planta Rich and Rich Composts (CMs) alone or in combination with seed coating by the antagonistic yeast Pichia guilliermondi before sowing on the severity of Rhizoctonia and Fusarium root rot and Fusarium wilt of faba bean was tested under greenhouse and field conditions. The tested isolates of yeast proved to be highly antagonistic to the pathogen in vitro. The test rates of CMs were equivalent to 2, 7, 10 and 14 ton/feddan in the greenhouse and 7 and 10 t/feddan in the field conditions. Uncomposted soil was used as a control. The results showed that the tested CMs have a suppressive effect on the severity of root rot and wilt diseases of faba bean under greenhouse and field conditions. The application of CMs (Planta Rich and Rich) alone at the rates equivalent to 2, 7, 10 and 14 t/feddan in greenhouse and 7 and 10 t/feddan in the field conditions to the soil infested with the tested pathogens reduced percentage of the tested diseases compared with uncomposted soil. Combined CMs treatments with yeast seed treatment increased the suppressive effect of CMs on the disease severity.     

Influence of the mycorrhizal fungus,Glomus coronatum,and soil phosphorus on infection and disease caused by binucleate Rhizoctonia and Rhizoctonia solani on mung bean (Vigna radiata)

Root-infecting fungal pathogens and also parasites, which do not cause major disease symptoms cause problems of contamination in pot cultures of arbuscular mycorrhizal (AM) fungi. We investigated the effect of the AM fungus, Glomus coronatum Giovannetti on disease caused by binucleate Rhizoctonia sp. (BNR) and R. solani in mung bean in the absence (P0) and presence (P1) of added soil phosphorus (P). When G. coronatum and BNR or R. solani were inoculated at the same time, G. coronatum improved the growth of the plants and reduced colonization of roots by BNR, but not by R. solani. R. solani reduced the growth of non-mycorrhizal mung bean in P0 soil 6 weeks after inoculation, whereas BNR had no effect on growth. G. coronatum reduced the severity of disease caused by BNR or R. solani on mung bean in both soil P treatments. When G. coronatum was established in the roots 3 weeks before BNR or R. solani was added to the potting mix, there was no significant effect of BNR or R. solani on growth of mung bean. Prior colonization by G. coronatum slightly reduced indices of disease caused by BNR or R. solani. In both experiments, addition of P stimulated plant growth and reduced the colonization of roots by BNR, but had little effect on disease severity. We conclude that the reduction of the effect of BNR or R. solani on mung bean could not be explained by improved P nutrition, but could be attributed to the presence of G. coronatum within and among the roots.     

Interactions between Sarocladium oryzae and stem attacking fungal pathogens of rice

In laboratory tests Sarocladium oryzae, the sheath rot pathogen of rice was found to inhibit the mycelial growth of other stem-attacking rice pathogens. Among those inhibited, Sclerotium oryzae and Gaeumannomyces graminis var. graminis were most sensitive while Pyricularia oryzae and Rhizoctonia solani were less sensitive. Tissue-based tests made with rice culm segments established that Sarocladium oryzae inhibits mycelial growth and delays sclerotium formation in R. solani. Cerulenin, the toxin produced by Sarocladium oryzae showed a toxicity pattern towards rice pathogens similar to that of Sarocladium oryzae. The stem rot pathogen, Sclerotium oryzae was most sensitive to cerulenin. In two greenhouse experiments, IR58 rice plants inoculated with Sarocladium oryzae alone or together with Sclerotium oryzae, G. graminis var. graminis or R. solani were found to have reduced plant height and increased tiller number. Sheath rot severity increased when Sarocladium oryzae was inoculated as a single pathogen or together with others. Sheath rot inoculation reduced stem rot in rice plants by 76 and 58%, respectively, in Experiment 1 and 2. By its known antagonistic interaction towards stem rot and crown sheath rot pathogens which are sensitive to it and by other unknown interactions, sheath rot emerges as the dominant disease.     

Use of three screening techniques for the evaluation of fungicides to control rhizoctonia root rot of wheat

Twenty fungicides were screened for their abilities to reduce the saprophytic growth of Rhizoctonia solani on agar and in soil, and to control root rot of wheat (Triticum aestivum) in pots of soil. Thirteen showed some activity against the in vitro growth of R. solani, the most promising being Sandoz 619F and flutolanil, although none of these reduced saprophytic growth of the fungus in soil. Only benomyl and thiabendazole, which were ineffective on agar and against disease in pots, reduced saprophytic growth in soil. None of the fungicides reduced the number of plants infected in the pot experiments and only six (flutriafol, diniconazole, Schering 539865, propiconazole, Bayer HWG1608 and flutolanil) of the thirteen active on agar (+ triadimefon which was not) reduced root rot severity. The results of this study indicate the differing reactions towards R. solani of chemicals in the three screening tests and confirm the potential for chemical control of rhizoctonia root rot of wheat.     

Expression of the bacteriophage T4 lysozyme gene in tall fescue confers resistance to gray leaf spot and brown patch diseases

Tall fescue (Festuca arundinacea Schreb.) is an important turf and forage grass species worldwide. Fungal diseases present a major limitation in the maintenance of tall fescue lawns, landscapes, and forage fields. Two severe fungal diseases of tall fescue are brown patch, caused by Rhizoctonia solani, and gray leaf spot, caused by Magnaporthe grisea. These diseases are often major problems of other turfgrass species as well. In efforts to obtain tall fescue plants resistant to these diseases, we introduced the bacteriophage T4 lysozyme gene into tall fescue through Agrobacterium-mediated genetic transformation. In replicated experiments under controlled environments conducive to disease development, 6 of 13 transgenic events showed high resistance to inoculation of a mixture of two M. grisea isolates from tall fescue. Three of these six resistant plants also displayed significant resistance to an R. solani isolate from tall fescue. Thus, we have demonstrated that the bacteriophage T4 lysozyme gene confers resistance to both gray leaf spot and brown patch diseases in transgenic tall fescue plants. The gene may have wide applications in engineered fungal disease resistance in various crops.     

The effect of soil compaction on the saprophytic growth of Rhizoctonia solani

The increase in bare patch of cereals associated with minimum tillage practices prompted an investigation of the relationship between soil compaction and saprophytic growth of Rhizoctonia solani. In soils wetter than 10 kPa there was a greater density of hyphae in compacted than in non-compacted soil. In relatively dry soil, however, there was wider exploration by hyphae in non-compacted than in compacted soil. The implications of these findings for disease management are discussed.     

Root Rot and Wilt of Kangaroo Paw (Anigozanthos manglesii) Caused by Pythium myriotylum (Drechs.) in Israel

A root rot and wilt disease of Anigozanthos manglesii (Kangaroo Paw) grown in greenhouses in Israel, for exporting as cut flowers to Europe, was characterized. Pythium myriotylum (Drechs.) and Rhizoctonia solani (Kühn) were the prevalent pathogens in diseased plants collected from commercial greenhouses. Fusarium oxysporum, Fusarium spp. and Myrothecium sp. were also isolated, but P. myriotylum or R. solani were not detected in samples from symptomless plants in tissue cultures (Australian origin) or plants at different stages in the nursery; non‐pathogenic F. oxysporum and Fusarium spp. were detected in several samples. In pathogenicity tests carried out in pots, plant mortality occurred 7 days after inoculation with P. myriotylum. In a field experiment carried out in methyl bromide‐fumigated soil, the incidence of dead plants following inoculation with P. myriotylum alone was 22% 10 days after inoculation, increasing to 78% after an additional 25 days. The incidence of dead plants following inoculation with R. solani alone was only 5% and in plants inoculated simultaneously with both pathogens, disease incidence was 88% 35 days after inoculation. Mortality reached 90–100% in plants inoculated with P. myriotylum, either singly or combined with R. solani 60 days after inoculation, whereas in plants inoculated with R. solani it was 5%. The maximum mortality in plants inoculated with R. solani was 25%, 76 days after inoculation. These results clearly demonstrate that P. myriotylum was the dominant pathogen in the root rot and wilt of A. manglesii.     

The long-term variation of Cd and Zn hyperaccumulation by Noccaea spp and Arabidopsis halleri plants in both pot and field conditions

Three Cd and Zn hyperaccumulating plant species Noccaea caerulescens Noccaea praecox and Arabidopsis halleri (Brassicacceae) were cultivated in seven subsequent vegetation seasons in both pot and field conditions in soil highly contaminated with Cd, Pb, and Zn. The results confirmed the hyperaccumulation ability of both plant species, although A. halleri showed lower Cd uptake compared to N. caerulescens. Conversely, Pb phytoextraction was negligible for both species in this case. Because of the high variability in plant yield and element contents in the aboveground biomass of plants, great variation in Cd and Zn accumulation was observed during the experiment. The extraction ability in field conditions varied in the case of Cd from 0.2 to 2.9 kg ha^?1 (N. caerulescens) and up to 0.15 kg ha^?1 (A. halleri), and in the case of Zn from 0.2 to 6.4 kg ha^?1 (N. caerulescens) and up to 13.8 kg.ha^?1 (A. halleri). Taking into account the 20 cm root zone of the soil, the plants were able to extract up to 4.1% Cd and 0.2% Zn in one season. However, cropping measures should be optimized to improve and stabilize the long-term phytoextraction potential of these plants.     

Efficacy of combined applications of antagonist bacteria and chemical resistance inducers for the management of Fusarium solani causing root rot in Withania somnifera

Application of Thiosalicylic acid+Bacillus cereus; O-Acetylsalicylic acid+Pseudomonas fluorescens reduced root rot severity by 85 and 88% and enhanced root yields by 358 and 419%, respectively, against Fusarium solani induced root rot disease in Withania somnifera. Reduction in disease severity was correlated with defence-related enzymes peroxidase, polyphenol oxidase and phenyl ammonium lyase.     

Diagnosis of zinc deficiency in seedlings of a tropical eucalypt (Eucalyptus urophylla S. T. Blake)

A glasshouse experiment was undertaken to establish the internal Zn requirement for shoot growth of Eucalyptus urophylla, a fast-growing commercial plantation species widely planted in tropical regions of the world. A Zn-deficient sand was supplemented with ten rates of Zn and seedlings were harvested after three months. In Zn-deficient plants the new growth was dwarfed with small, necrotic leaves and short internodes. Foliar Zn concentrations declined markedly with leaf age in both Zn-deficient and Zn-adequate plants. The critical Zn concentration for the diagnosis of Zn deficiency also fell with leaf age. Zinc concentrations in the youngest fully expanded leaf ranged from 8–11 g Zn g^–1 dry weight in plants with severe symptoms to 30–37 g Zn g^–1 dry weight in non-deficient plants. The critical Zn concentration for the diagnosis of Zn deficiency at 90% of maximum shoot growth in the same leaf was 21 g Zn g^–1 dry weight. This value is nearly twice that reported for several other species of eucalypts and may indicate a higher internal demand for Zn in tropical than in temperate eucalypts.