Incidence of Three Major Cassava Diseases on Local Susceptible Cassava Cultivars at Three Planting Dates

Incidence of African cassava mosaic (ACMD), cassava bacterial blight (CBB), and cassava anthracnose (CA) on local susceptible cassava cultivars planted in December, February, and April was investigated. December cassava planting had a higher incidence of ACMD, CBB, and CA diseases compared with February and April plantings. CA seemed to be more prevalent at an older stage (8 months old) of plant growth. April planting had significantly lower incidence of major cassava diseases, and higher mean tuber, yield than the same cassava cultivars planted earlier. The length of exposure of cassava plants to the inocula and/or to the vector during the rainy season determines the occurrence of diseases. In the absence of resistant cassava cultivars and through, partial disease escape of susceptible cultivars from ACMD, CBB, and CA, planting the same cultivars in April can still produce a profitable cassava crop.     

The holoparasitic endophyte Bdallophyton americanum affects root water conductivity of the tree Bursera simaruba

It has been reported that parasitic vascular plants (hemiparasites and holoparasites) may affect host fitness, but the effects produced by root endophyte holoparasitic species on its host have not been documented. Here the effect of the holoparasitic endophyte Bdallophyton americanum (R. Br.) Harms on the root conductivity of Bursera simaruba (L.) Sarg. was studied. Parasitized and non-parasitized root segments were sampled in the rainy and dry seasons in a dry coastal forest in central Veracruz, Mexico. Root diameter, hydraulic (K _h) and specific conductivity (K _s = K _h/root transverse area), percent loss of conductivity and reproductive specific conductivity (K _h /inflorescence or infructescense dry weight) data were obtained. The diameter and number of conductive and non-conductive vessels were recorded in parasitized and non-parasitized root segments in the dry season. Root diameters were not different between root types and seasons, but root specific conductivity was different both between seasons and root types. Specific conductivity on parasitized roots was 61% (wet season) and 85% (dry season) lower than that recorded for non-parasitized roots in the wet season. Root hydraulic conductivity was positively related with the biomass of reproductive structures of B. americanum in the wet season. The parasite appears to alter the xylem morphogenesis of the host, reducing vessel number by 40%, but not plugging or otherwise harming the conductive vessels, and resulted in no change in vessel diameter. Contrary to what has been reported to occur in some plant stems infected with hemiparasitic mistletoes, B. americanum decreases but does not eliminate conductivity to the distal plant parts.     

Organic Matter Stimulates Bacteria and Arbuscular Mycorrhizal Fungi in Bauhinia purpurea and Leucaena diversifolia Plantations on Eroded Slopes in Nepal

Erosion resulting from landslides is a serious problem in mountainous countries such as Nepal. To restore such sites it is essential to establish plant cover that protects the soil and reduces surface erosion. Mycorrhizal fungi growing in symbiosis with plants are essential in this respect because they improve both plant nutrient uptake and soil structure. We investigated the influence of organic matter and P amendment on recently produced biomass of bacteria and arbuscular mycorrhizal (AM) fungi in eroded slopes in Nepal. Eroded soil mixed with different types of organic matter or P was placed in mesh bags, which were buried around trees of Bauhinia purpurea and Leucaena diversifolia between June 2003 and December 2003 (the wet season) or between December 2003 and June 2004 (the dry season). Signature fatty acids were used to determine bacterial and AM fungal biomass after the 6‐month intervals. The amount and composition of AM fungal spores were analyzed in the mesh bags from the dry season. More microbial biomass was produced during the wet season than during the dry season. Furthermore, organic matter addition enhanced the production of AM fungal and bacterial biomass during both periods. The positive influence of organic matter addition on AM fungi could be an important contribution to plant survival in plantations on eroded slopes. Different AM spore communities and bacterial profiles were obtained with different organic amendments and this suggests a possible way of selecting for specific microbial communities in the management of eroded sites.     

Seasonal ectomycorrhizal fungal biomass development on loblolly pine (Pinus taeda L.) seedlings

Ergosterol, a membrane sterol found in fungi but not in plants, was used to estimate live mycelial biomass in ectomycorrhizae. Loblolly pine (Pinus taeda L.) seeds were sown in April 1993 and grown with standard nursery culture practices. Correlations between total seedling ergosterol and visual assessment of mycorrhizal colonization were high during July and August but low as ectomycorrhizal development continued into the growing season. Percentages of mycelial dry weight over lateral roots decreased from 9% in July to 2.5% in November because seedling lateral root dry weight accumulated faster than mycelial dry weight. Total ergosterol per seedling increased from July through February. As lateral root dry weight ceased to increase during winter months, ectomycorrhizal mycelia became the major carbohydrate sink of pine seedlings. No distinctive seasonal pattern of soil ergosterol content was observed. The impact of ectomycorrhizal fungi on plant carbohydrate source-sink dynamics can be quantitatively estimated with ergosterol analysis but not with conventional visual determination.     

Phytoextraction of heavy metals by two Salicaceae clones in symbiosis with arbuscular mycorrhizal fungi during the second year of a field trial

We evaluated the potential of Salix viminalis (5027) and Populus × generosa for the phytoextraction of heavy metals (HM) inoculated or not with an arbuscular mycorrhizal (AM) fungus Glomus intraradices during a second year of growth in a randomized complete block field trial on a slightly contaminated site. Both plant clones produced high aboveground biomass yields, however P. × generosa produced significantly more biomass than S. viminalis. The two plant clones accumulated high concentrations of Cd and Zn in their shoots, while Cu and Pb were stored in their roots. In general, S. viminalis accumulated higher concentrations of HM. The inoculation of G. intraradices in the previous year did not influence plant clones’ biomass yields during the second growing season. However, Cu and Cd translocation to shoots was limited, and Cu was preferentially concentrated in roots of inoculated plants, compared to non-inoculated plants, which were also colonized by native AM fungi taxa. Efficiency of S. viminalis and P. × generosa for Cd and Zn rehabilitation in slightly contaminated soil has been demonstrated, but mycorrhizal inoculation did not significantly increase HM extraction.     

Cadmium accumulation in sunflower plants influenced by arbuscular mycorrhiza

In order to investigate the cadmium (Cd) accumulation patterns and possible alleviation of Cd stress by mycorrhization, sunflower plants (Helianthus annuus L.) were grown in the presence or absence of Cd (20 micromol L(-1)) and inoculated or not inoculated with the arbuscular mycorrhizal fungus (AMF) Glomus intraradices. No visual symptoms of Cd phytotoxicity were observed; nevertheless, in non-mycorrhizal plants the presence of Cd decreased plant growth. The addition of Cd had no significant effect on either mycorrhizal colonization or the amount of extra-radical mycelia that was produced by the AMF. Cd accumulated mainly in roots; only 22% of the total Cd absorbed was translocated to the shoots, where it accumulated to an average of 228 mg Cd kg(-1). Although the shoot-to-root ratio of Cd was similar in both the AMF inoculated and non-inoculated plants, the total absorbed Cd was 23% higher in mycorrhizal plants. Cd concentration in AMF extra-radical mycelium was 728 microg g(-1) dry weight. Despite the greater absorption of Cd, mycorrhizal plants showed higher photosynthetic pigment concentrations and shoot P contents. Cd also influenced mineral nutrition, leading to decreased Ca and Cu shoot concentrations; N, Fe and Cu shoot contents; and increased S and K shoot concentrations. Cd induced guaiacol peroxidase activity in roots in both mycorrhizal and non-mycorrhizal plants, but this increase was much more accentuated in non-mycorrhizal roots. In conclusion, sunflower plants associated with G. intraradices were less sensitive to Cd stress than non-mycorrhizal plants. Mycorrhizal sunflowers showed enhanced Cd accumulation and some tolerance to excessive Cd concentrations in plant tissues.     

Temporal and morphological factors related to mineral composition in Myriophyllum heterophyllum michx

The emergent floral stem, apex, sub-apex, mid-stem, lower stem, and roots of Myriophyllum heterophyllum Michx were analyzed for ash, P, Na, K, Ca, Mg, Fe, Mn, Zn, Cu and Pb. Samples were collected 14 times from June 1979 through July 1980 from Lake Winnipesaukee, New Hampshire, U.S.A.The plant segments sampled differed significantly in mineral content, but seasonal pulses for each mineral were usually in synchrony between the different segments sampled. Sodium (range, 0.8–2.7% dry wt.) and potassium (range, 1.4–3.3% dry wt.) were the dominant elements in the submersed stem. The dominant element in the floral spike was Ca (range, 2.4–4.1% dry wt.) and in the roots was Fe (range, 2.8–8.0% dry wt.). Sodium concentrations were greater in the submersed stem (range, 0.8–2.7% dry wt.), relative to the roots (range, 0.5–0.9% dry wt.) and flora stem (range, 0.3–1.1% dry wt.). Lead concentrated in the roots significantly more (range, 0.0040–0.0115% dry wt.), than in other plant parts (range, 0–0.0037% dry wt.).The submersed stem had spring or summer peaks in ash, P, Na, K, Ca, Mg, and Zn content, whereas Fe and Mn exhibited late winter to early spring peaks. No evidence of nutrient storage during the non-growing season was evident in the segments sampled. Explanations for the seasonal and morphological differences observed are presented.     

Seasonal variation in the nutrient profile of <Emphasis Type="Italic">Arthrospira fusiformis</Emphasis> biomass harvested from an Ethiopian soda lake,Lake Chitu

The extent of seasonal variation in the nutrient profile of Arthrospira biomass harvested from Lake Chitu was investigated to evaluate the variability of the quality of the product over a period of a year. Protein content varied from 47.9 to 55.7% for wet season biomass samples and from 39.2 to 40.8% for dry season samples. Dry season samples were characterized by relatively higher carbohydrate values (38.0–41.3%). Higher proportion of amino acids and unsaturated fatty acids were recorded for biomass harvested in wet season. Similarly, higher contents of phytonutrients (pigments) were recorded for wet season biomass samples: chlorophyll a (8.2–10.3 mg g^?1), phycobiliproteins (104.1–120.7 mg g^?1), total carotenoids (3.17–4.31 mg g^?1), and β-carotene (1.24–1.61 mg g^?1). The contents of Na and K were higher for a dry season biomass whereas other major (Ca, P, Mg) and trace (Mn, Fe, Cu, Zn, Se) minerals were found relatively in higher quantities in a wet season biomass. The nutritional composition of Arthrospira from Lake Chitu was found to be relatively comparable to that found in commercial Arthrospira products in the market. The significance of the findings is discussed in relation to potential sustainable production of Arthrospira biomass from this lake.     

丛枝菌根真菌菌丝体吸附重金属的潜力及特征

应用玻璃珠分室培养系统获得丛枝菌根真菌材料,研究了离体真菌菌丝体对pH缓冲体系中Zn、Cd和Mn等金属离子的吸附特征。试验结果表明,真菌菌丝体对各金属离子吸附能力差异显著,对Cd最强,Zn次之,Mn最弱。试验条件下,菌体可分别吸附相当于自身干物重1.6%的Mn、2.8%的Zn和13.3%的Cdo吸附于菌丝体的Cd2+绝大部分可以被Ca2+交换吸附。另外研究了宿主植物根系对Cd的吸附作用,证实菌根真菌侵染改变了根系的吸附特性,相对于非菌根根系,菌根的CEC较高,对Cd的吸附能力较强。试验结果为重金属污染条件下丛枝菌根强化根系的屏障作用提供了直接证据。     

水分胁迫下丛枝菌根真菌对枳实生苗生长和渗透调节物质含量的影响

采用盆栽试验研究了水分胁迫下接种丛枝菌根真菌摩西球囊霉(Glomaus mosseae)对枳[Poncirustrifoliat(L.)Raf.]实生苗的生长和渗透调节物质含量的影响.结果表明,在土壤含水量为20%、16%和12%条件下,接种G.mosseae能够增加植株的生长(株高、茎粗、叶面积、地上部干重、地下部干重和植株干重),促进植株根系活跃吸收面积和根际土壤有效磷的吸收,提高叶片和根系可溶性糖含量的积累,降低叶片脯氨酸含量,增强植株的水分利用效率(达20%～40%),使枳实生苗的抗旱能力得到增强.土壤含水量为20%和16%条件下接种G.mosseae对植株的效果较土壤含水量为12%条件下更显著.12%的土壤含水量严重抑制Gmosseae的侵染,说明丛枝菌根侵染程度轻,其对植物的效果也差.     

Viruses Associated with Cassava Mosaic Disease in Senegal and Guinea Conakry

A survey in Senegal and Guinea Conakry established the presence and incidence of cassava mosaic virus disease (CMD) in both countries. CMD occurred in all the fields surveyed, although its incidence was higher in Senegal (83%) than in Guinea (64%). Populations of the whitefly vector, Bemisia tabaci, were low in both countries averaging 1.7 adults per shoot in Guinea and 3.2 in Senegal. Most infections were attributed to the use of infected cuttings, 86 and 83% in Senegal and Guinea, respectively, and there was no evidence of rapid current‐season, whitefly‐borne infection at any of the sampled locations. Disease severity was generally low in the two countries and averaged 2.5 in Guinea and 2.3 in Senegal. No plants with unusually severe CMD symptoms characteristic of the CMD pandemic in East and Central Africa were observed. Restriction fragment length polymorphism (RFLP)‐based diagnostics revealed that African cassava mosaic virus (ACMV) is exclusively associated with CMD in both the countries. Neither East African cassava mosaic virus (EACMV), nor the recombinant Uganda variant (EACMV‐UG2) was detected in any sample. These survey data indicate that CMD could be effectively controlled in both countries by phytosanitation, involving the use of CMD‐free planting material and the removal of diseased plants.     

Mycorrhizas in the Kakadu region of tropical Australia

This research represents the first part of a study which aimed to characterize the role of mycorrhizal associations in undisturbed and disturbed habitats in the Alligator Rivers Region of the Northern Territory of Australia. This is a seasonally dry tropical region with a climate consisting of a long dry season and a monsoonal wet season. Intact soil cores were sampled from 22 sites in this region, representing eucalypt savanna woodland, wetland, rocky hill and rainforest habitats. Clover, sorghum and eucalypt seedlings were grown in these cores in bioassays to measure the inoculum potential of vesicular-arbuscular mycorrhizal (VAM) and ectomycorrhizal (ECM) fungi. Propagules of VAM fungi were concentrated in the surface horizon, and were not adversely affected by 6 months dry storage of soil. Bioassays detected VAM fungus propagules at all sites, but these were less numerous in three sites with sparse herbaceous vegetation (a shrub-dominated woodland site, a sandstone area and a disturbed gravel pit without topsoil), than in other woodland sites. Propagules of VAM fungi were particularly numerous in soil from a rainforest habitat, which had much denser plant cover than any of the savanna sites. Propagules of ECM fungi colonized eucalypt seedling roots in some cores from all sites, except two wetland areas and a disturbed area without eucalypt trees. Physical and chemical properties of soils varied between sites and some properties (texture, organic carbon, etc.) were correlated with the inoculum potential of VAM fungi.     

Screening landraces for additional sources of field resistance to cassava mosaic disease and green mite for integration into the cassava improvement program

Twelve cassava landraces were evaluated for sources of resistance genes to diseases and pests of major economic importance in Africa. The objective was to assess their levels of field resistance to mosaic disease （ACMD）, bacterial blight （CBB）, anthracnose （CAD）, and green mite （CGM）, compared to TMS30572, an elite cultivar widely adopted in Africa. Considerable genotypic variation was observed among cultivars for resistance to ACMD and CGM but not for CBB and CAD. The lowest mean incidence of 12% and severity of 1.8 on a scale of 1-5 for ACMD was recorded for Atu, a landrace with farmer acceptable qualities. In comparison, the improved cultivar, TMS 30572, had a mean disease incidence of 72% and a severity score of 2.8. Another landrace, MS-20 had the lowest CGM damage score （2.1） while TMS 30572 emerged as one of the susceptible cultivars with a damage score of 3. Additional sources of resistance to ACMD and CGM that may possibly be better than the popular improved cultivar, TMS 30572, were identified in this study. These could serve as novel sources of additional genes to complement existing resources for elite cassava breeding in Africa.     

Abundance and Leaf Damage Caused by Gall‐Inducing Insects in a Mexican Tropical Dry Forest1

We evaluated the temporal and spatial patterns of abundance and the amount of damage caused by gall‐inducing insects (GII) in deciduous and riparian habitats in a seasonal tropical dry forest in Mexico. Plants occurring in these habitats differ in their phenology and moisture availability. Deciduous habitats are seasonal and xeric, while riparian habitats are aseasonal and mesic. We found 37 GII species and each one was associated with a specific plant species. In total, 19 species (51.3%) were present in deciduous habitats, 13 species (35.2%) in riparian habitats, and only 5 species (13.5%) occurred in both. Abundance and leaf damage by GII were greater in deciduous than in riparian habitats during the wet season. For each GII species that occurred in both habitats, host plant species supported greater abundance and leaf damage by GII in deciduous habitats during the wet season. These results indicate a greater association of GII species with host plants in deciduous than in riparian habitats during the wet season. In riparian habitats, 11 plant species (61.1%) had greater density of GII in the dry than in the wet season. Similarly, leaf damage by GII was significantly greater in the dry than in the wet season in riparian habitats for 12 plant species (66.7%). Dry forest plants of riparian habitats presented two peaks of leaf‐flushing: GII colonized leaves produced in the first peak at the beginning of the wet season, and accumulated or recolonized leaves in the second peak at the beginning of the dry season. The levels of leaf damage by GII detected in this study in the rainy season were considerably higher than those obtained for folivorous insects in other neotropical forests, suggesting that this GII guild might have an important impact on their host plant species in this tropical community.     

Effects of liming and mycorrhizal colonization on soil phosphate depletion and phosphate uptake by maize (Zea mays L.) and soybean (Glycine max L.) grown in two tropical acid soils

The effects of liming and inoculation with the arbuscular mycorrhizal fungus, Glomus intraradices Schenck and Smith on the uptake of phosphate (P) by maize (Zea mays L.) and soybean (Glycine max [L.] Merr.) and on depletion of inorganic phosphate fractions in rhizosphere soil (Al-P, Fe-P, and Ca-P) were studied in flat plastic containers using two acid soils, an Oxisol and an Ultisol, from Indonesia. The bulk soil pH was adjusted in both soils to 4.7, 5.6, and 6.4 by liming with different amounts of CaCO₃.In both soils, liming increased shoot dry weight, total root length, and mycorrhizal colonization of roots in the two plant species. Mycorrhizal inoculation significantly increased root dry weight in some cases, but much more markedly increased shoot dry weight and P concentration in shoot and roots, and also the calculated P uptake per unit root length. In the rhizosphere soil of mycorrhizal and non-mycorrhizal plants, the depletion of Al-P, Fe-P, and Ca-P depended in some cases on the soil pH. At all pH levels, the extent of P depletion in the rhizosphere soil was greater in mycorrhizal than in non-mycorrhizal plants. Despite these quantitative differences in exploitation of soil P, mycorrhizal roots used the same inorganic P sources as non-mycorrhizal roots. These results do not suggest that mycorrhizal roots have specific properties for P solubilization. Rather, the efficient P uptake from soil solution by the roots determines the effectiveness of the use of the different soil P sources. The results indicate also that both liming and mycorrhizal colonization are important for enhancing P uptake and plant growth in tropical acid soils.     

The Relative Resistance of Cassava Cultivars to African Cassava Mosaic Disease (ACMD) as Determined by Two Methods: Rank-Sum and the Area Under the Disease Progress Curve

Twenty-five newly bred improved cassava cultivars, twenty-three improved from the International Institute of Tropical Agriculture and two local cultivars were evaluated for their relative resistance to African cassava mosaic begomovirus disease (ACMD) at Ibadan, in an area of high disease pressure representative of the forest/savanna transition zone of Nigeria. These cultivars were exposed to natural infection by the viruliferous whiteflies ( Bemisia tabaci Gennadius) and the disease incidence (DI) and index of symptom severity (ISS) were assessed for all clones. Results for the Rank-sum (i.e., sum of ranks for DI and ISS for each cultivar) and the area under the disease progress curve (AUDPC) were used to assess the relative resistance of the cassava clones. Those that showed low Rank-sum and AUDPC values were rated 'moderately resistant (MR)', 'resistant ( R )', and 'highly resistant (HR)' to ACMD depending on their respective values and deviation from the mean distribution curve. Clones M94/0121 and 94/0239 were rated HR under the two methods. Clone M94/0583 was rated HR under the AUDPC with a deviation from the mean distribution curve of m 2.00 while it was rated R under the Rank-sum method with a deviation from the mean distribution of m 1.99. Also plants of clones ISU and TMS 30572 were rated highly susceptible (HS) under both methods. Clone TME-1 was intermediate between Moderately resistant (MR) and Moderately susceptible (MS) under the AUDPC method with a deviation from the mean distribution of 0.00 but observed to be MS under the Rank-sum method with a deviation of + 0.2. The two methods of evaluation gave similar results as revealed by Spearman rank correlation ( r equals; 0.99, P <0.01). However, the AUDPC method is less cumbersome compared to the Rank-sum method. None of the clones was observed to be immune to the disease.     

Diet of Zonocerus variegatus (Linné, 1758) (Orth., Acrididae) in cassava fields in Bénin

Abstract:  This paper presents the results of a study on the diet of Zonocerus variegatus in cassava agro-ecosystems at Allada, south Benin. The diet was determined by the analysis of plant fragments in the faeces of insects sampled in the fields. The grasshoppers studied consumed 21 plant species among the 81 found in the fields. Cassava, maize, Mallotus oppositifolius and Chromolaena odorata were the major food plants for Z. variegatus in this system. The presence of Z. variegatus in different fields was observed depending on the season and the presence of certain plant species. There was no relationship between plant frequency in the diet and in the field. Cassava was the preferred food resource for mature nymphs and adults. During the dry season, young nymphs stayed near the hatching sites and where good food resources were available. At this time, the grasshoppers were present in a field where cassava was not abundant but where maize and weeds were available. During the dry season, Z. variegatus had a broader diet than during the wet season. This contradicts the current hypothesis that the water content of plants and the floral diversity determines food preference. The dry season is also the time when C. odorata leaves were found in the faeces of both adults and nymphs. These observations made in West Africa are similar to those made in Central Africa, particularly for the number of plant species found in the faeces, 21 (26% of all plant species found in the fields) in Bénin and 17 (23%) in the Congo. The increase of the sample size and the great differences between the ecoregional situation of the study sites do not substantially change the number of plant species included in the diets. In the area covered by this study, C. odorata is not the major substitute for cassava during the dry season, although it remains a major food source and an especially visible food resource.     

Environmental constraints on the foraging behaviour of a dwarf antelope (Madoqua kirkii)

Dik-diks (Madoqua sp.) inhabit semi-arid regions and experience very different conditions of food availability and quality between wet and dry seasons. By comparing the behaviour of dik-diks between these two seasons, we identified environmental constraints affecting their feeding strategies. In both seasons foraging time was limited by high mid day temperatures. In the wet season a high intake rate compensates for the loss in foraging time, but in the dry season water and protein become limiting. To meet minimum daily water requirements in the dry season dik-diks fed on plant species that they avoided during the wet season. Analysis at the plant species level showed higher species selectivity in the wet season than in the dry season. In a multiple regression analysis food species preferences were best explained by relative abundance and water content in the dry season, and by dry matter content in the wet season. In the wet season the daily dry-matter intake of dik-diks in the field was only about 10% higher than the theoretically predicted minimum for a ruminant of this body weight, while protein and water intake were about 3 times as high. This suggests that the most limiting dietary component in the wet season is energy. In the dry season the daily intake of all dietary components is lower than the theoretical minimum required, and also lower than the values suggested by laboratory studies of dik-diks. This dry season deficit is presumably met from body reserves. Dry season water intake was approximately 30% of the intake observed in laboratory studies indicating that dikdiks are even better adapted to arid conditions than suggested by physiological experiments.     

Application of a Simplified Molecular Protocol to Reveal Mixed Infections with Begomoviruses in Cassava

Samples of cassava leaves exhibiting severe symptoms of cassava mosaic disease (CMD) were collected with the PhytoPASS kit in fields surrounding the city of Bujumbura (Burundi). These materials were then sent to Belgium for polymerase chain reaction determination of the CMD begomoviruses inducing the observed symptoms. Different pairs of specific primers were used to amplify DNA sequences specific to African cassava mosaic virus (ACMV), East African cassava mosaic virus (EACMV), East African cassava mosaic Cameroon virus (EACMCV), East African cassava mosaic Malawi virus (EACMMV), East African cassava mosaic Zanzibar virus (EACMZV), the Uganda variant of East African cassava mosaic virus (EACMV-UG) and South African cassava mosaic virus (SACMV). It was revealed that mixed infections were prevailing in the analyzed materials. Most of the samples submitted to this analysis were found to be co-infected by three different begomoviruses (ACMV + EACMV + EACMV-UG). The so revealed mixed infections could explain the high severity of CMD symptoms noticed on cassava in the region of Bujumbura while the diversity within the CMD causal agents illustrates the importance to take this parameter into consideration for a successful use of plant genetic resistance to control the disease.     

Changes in carbon allocation and expression of carbon transporter genes in Betula pendula Roth. colonized by the ectomycorrhizal fungus Paxillus involutus (Batsch) Fr.

Comparative analyses of aspects of the carbon (C) physiology and the expression of C transporter genes in birch (Betula pendula Roth.) colonized by the ectomycorrhizal fungus Paxillus involutus (Batsch) Fr. were performed using mycorrhizal (M) and non‐mycorrhizal (NM) plants of similar foliar nutrient status. After six months of growth, the biomass of M plants was significantly lower than that of NM plants. Diurnal C budgets of both sets of plants revealed that M plants exhibited higher rates of photosynthesis and root respiration expressed per unit dry weight. However, the diurnal net C gain of M and NM plants remained similar. Ectomycorrhizal roots contained higher soluble carbohydrate pools and increased activity of cell wall invertase, suggesting that additional C was allocated to these roots and their ectomycorrhizal fungi consistent with an increased sink demand for C due to the presence of the mycobiont. In M roots, the expression of two hexose and one sucrose transporter genes of birch were reduced to less than one‐third of the expression level observed in NM roots. Analysis using a probe against the birch ribosomal internal transcribed spacer region revealed that M roots contained 22% less plant RNA than NM roots. As the expression of birch hexose and sucrose transporter genes was reduced to a much greater extent, this suggests that these specific genes were down‐regulated in response to alterations in C metabolism within M roots.