The effect of root pruning on the arbuscular mycorrhizal symbiosis in grapevine rootstocks

Background Arbuscular mycorrhizal fungi provide benefits to plants, especially under stressful growing conditions. These symbiotic fungi can be applied as biofertilizers prior to transplant in order to increase establishment success in the field. Roots are often trimmed at the time of transplant to reduce the probability of J-rooting, the upward orientation of roots within a planting hole which can lead to root death and disease. The effect of root trimming on the mycorrhizal symbiosis is unknown. It is possible that trimming may remove the active mycelium, nullifying the effect of inoculation. We conducted a greenhouse study to test the effect of root trimming on the mycorrhizal symbiosis in grapevine. Results The mycorrhizal symbiosis persisted after root trimming. Trimming reduced the abundance of AM fungi in older roots. The fungi were able to recolonize the new roots in trimmed vines, and these roots had more arbuscules compared to older roots, which had mostly vesicles. Trimmed vines had lower shoot, but not root, biomass. Conclusions The mycorrhizal symbiosis persisted in the roots, despite trimming, likely due to fungal structures in older, untrimmed roots serving as propagules. We conclude that inoculation with AM fungi prior to field transplant is robust to root trimming, at least for the isolate examined in this study.

Spatial Distribution of Mealybugs (Hemiptera: Coccomorpha: Coccoidea) in the Root System of Pruned and Non-pruned Coffea arabica Trees

Root mealybugs are important pests of coffee in many parts of the world. Despite the importance of these insects very little is known about their biology, ecology, and coffee susceptibility at different developmental stages. This study determined the effects of coffee tree pruning on root biomass, the number and within-plant spatial distribution of root-associated mealybugs, and weed-mealybug associations near coffee root systems. We made destructive root sampling of pruned and non-pruned coffee trees every 3 mo during the first-year post-pruning. In each sampling, we quantified coffee root biomass, and the number and spatial distribution of mealybug genera. Mealybugs feeding on weed roots were sampled every 2 wk for the duration of 10 wk. We identified the mealybug genera Puto Signoret (Hemiptera: Putoidae), Dysmicoccus Ferris (Hemiptera: Pseudococcidae), Ripersiella Tinsley (Hemiptera: Rhizoecidae), Rhizoecus Künckel (Hemiptera: Rhizoecidae), and Geococcus Green (Hemiptera: Pseudococcidae) associated with coffee roots. Species from Puto and Ripersiella were the most abundant representing 91% of the total mealybugs found in non-pruned trees, and 75% of the ones found in pruned trees. Coffee tree pruning reduced root biomass by 60% and mealybug numbers during the first 9 mo post-pruning. Mealybugs associated with a variety of weeds growing near coffee roots increased their number during the first 6 wk after coffee pruning. Our results suggest that coffee tree pruning causes a high percentage of root death, which correlates with a decrease of root-associated mealybugs that probably migrate to weed roots. This study provides basic information for designing mealybug control strategies in pruned coffee trees. Las cochinillas de las raíces son plagas importantes del café en varias partes del mundo. A pesar de la importancia de estos insectos se conoce muy poco sobre su biologia, ecología y sobre la susceptibilidad de plantas de café en diferentes etapas de desarrrollo. Este estudio determinó el efecto de la poda de árboles de café en la biomasa de raíces, el número y la distribución espacial in-planta de cochinillas asociadas a las raíces, y asociaciones de cochinillas con arvenses en el plato de los árboles de café. Se realizaron muestreos destructivos de raíces en árboles de café podados y no podados cada tres meses durante el primer año después de la poda. Las cochinillas alimentándose de raíces de arvenses fueron muestreadas cada dos semanas durante diez semanas. En cada muestreo se cuantificó la biomasa de raíces de café, se identificó el género de cochinillas, su cantidad y su distribución espacial. Se identificaron los géneros de cochinillas Puto Signoret (Hemiptera: Putoidae), Dysmicoccus Ferris (Hemiptera: Pseudoccocidae), Ripersiella Tinsley (Hemiptera: Rhizoecidae), Rhizoecus Künckel (Hemiptera: Rhizoecidae), y Geococcus Green (Hemiptera: Pseudococcidae) asociados a raíces de café. Especies de Puto y Ripersiella fueron las más abundantes representando el 91% del total de cochinillas encontradas en árboles no podados y el 75% de aquellas encontradas en árboles podados. La poda de árboles de café redujo la biomasa de raíces en un 60% y el número de cochinillas durante los primeros nueve meses después de la poda. Las cochinillas asociadas a varias especies de arvenses presentes en los platos de los árboles de café incrementaron su número durante las primeras seis semanas después de la poda de los árboles. Los resultados de este estudio sugieren que la poda de árboles de café causa una alta muerte de raíces, lo cual se correlaciona con una reducción en las cochinillas quienes probablemente migran hacia las raíces de las arvenses. Este estudio proporciona información básica para el diseño de estrategias de control de cochinillas en árboles de café podados.

Interactions between waterlogging and ray blight in pyrethrum

The effects of waterlogging, alone and combined with ray blight disease (caused by Stagonosporopsis tanaceti), on pyrethrum (Tanacetum cinerariifolium) plant growth were quantified in glasshouse trials. Six pyrethrum cultivars were initially studied for their response to 6 days of waterlogging and their recovery from waterlogging during 26 days post-waterlogging. Waterlogging caused substantial root death and leaf wilting and accelerated senescence in all cultivars. Root growth was 80% more reduced than shoot growth. Cultivar ‘F’ showed significantly higher root porosity and growth following waterlogging than other cultivars. In contrast, cv. ‘C’ had the greatest growth reduction from waterlogging and poor root-system recovery after waterlogging. Plants of cvv. C and F inoculated with S. tanaceti and then waterlogged were more significantly affected than were those exposed to waterlogging only. For both cultivars, shoot growth under the combined treatment, relative to initial growth, recovered up to 25%, but root growth suffered irreversible damage. The combined treatment decreased the number of stems by 39% compared with waterlogging alone after the post-waterlogging period. In conclusion, pyrethrum cultivars showed differential reactions to waterlogging; but growth in all cultivars was seriously affected by a combination of waterlogging and infection by ray blight.

Root growth of lupins is more sensitive to waterlogging than wheat

In south-west Australia, winter grown crops such as wheat and lupin often experience transient waterlogging during periods of high rainfall. Wheat is believed to be more tolerant to waterlogging than lupins, but until now no direct comparisons have been made. The effects of waterlogging on root growth and anatomy were compared in wheat (Triticum aestivum L.), narrow-leafed lupin (Lupinus angustifolius L.) and yellow lupin (Lupinus luteus L.) using 1 m deep root observation chambers. Seven days of waterlogging stopped root growth in all species, except some nodal root development in wheat. Roots of both lupin species died back progressively from the tips while waterlogged. After draining the chambers, wheat root growth resumed in the apical region at a faster rate than well-drained plants, so that total root length was similar in waterlogged and well-drained plants at the end of the experiment. Root growth in yellow lupin resumed in the basal region, but was insufficient to compensate for root death during waterlogging. Narrow-leafed lupin roots did not recover; they continued to deteriorate. The survival and recovery of roots in response to waterlogging was related to anatomical features that influence internal oxygen deficiency and root hydraulic properties.

A Phytoplasma Associated with an Outbreak of an Unusual Disease of Chrysanthemum in China in 2008

In August of 2008, a disease of chrysanthemum (Dendranthema morifolium (Ramat.) Tzvel) caused losses of 70 to 80% in one of the largest chrysanthemum gardens in Yangling, Shanxi Province, China. Chrysanthemum plants in nearby areas also were affected to various degrees. Symptoms included flattened stems, shortening of internodes, yellowing of leaf margins, root death, and dwarfing of plants. Affected plants eventually collapsed. On the basis of these symptoms, a phytoplasma was suspected. Total nucleic acids were extracted from 0.5 g of phloem tissue from stems of eight symptomatic and eight asymptomatic plants by the cetyltrimethylammoniumbromide (CTAB) method (1). To amplify phytoplasma DNA, primer pairs R16mF2/R16mR1, followed by R16F2n/R16R1 (2), were used in a nested PCR. A final amplicon product (1.2 kb) was obtained from all symptomatic plants but not from asymptomatic ones. Restriction fragment length polymorphism (RFLP) analyses of R16F2n/R16R1 amplicons with MseI, AluI, HhaI, HaeIII, KpnI, RsaI, and HpaII endonucleases indicated that all symptomatic plants, but none of the asymptomatic plants, contained a phytoplasma strain of group 16SrI, subgroup B (3). A search of rDNA sequences in GenBank revealed a similarity (>99%) to aster yellow phytoplasma, 16SrI group, thereby confirming strain identity based on RFLP analysis. These results indicate the disease of chrysanthemum is associated with a phytoplasma related to the aster yellow phytoplasma group. Sequences were deposited in GenBank (Accession No. FJ543467). A vector of this phytoplasma in chrysanthemum has not been identified. References: (1) E. Angelini et al. Vitis 40:79, 2001. (2) D. E. Gundersen and I.-M. Lee. Phytopathol. Mediterr. 35:144, 1996. (3) I. M. Lee et al. Int. J. Syst. Evol. Microbiol. 48:1153, 1998.

Potential contribution by cotton roots to soil carbon stocks in irrigated Vertosols

The well-documented decline in soil organic carbon (SOC) stocks in Australian cotton (Gossypium hirsutum L.) growing Vertosols has been primarily analysed in terms of inputs from above-ground crop residues, with addition to soil C by root materials being little studied. Potential contribution by cotton roots to soil carbon stocks was evaluated between 2002 and 2008 in 2 ongoing long-term experiments near Narrabri, north-western New South Wales. Experiment 1 consisted of cotton monoculture sown either after conventional tillage or on permanent beds, and a cotton–wheat (Triticum aestivum L.) rotation on permanent beds; Experiment 2 consisted of 4 cotton-based rotation systems sown on permanent beds: cotton monoculture, cotton–vetch (Vicia villosa Roth.), cotton–wheat, and cotton–wheat–vetch. Roundup-Ready™ (genetically modified) cotton varieties were sown until 2005, and Bollgard™ II-Roundup Ready™-Flex™ varieties thereafter. Root growth in the surface 0.10 m was measured with the core-break method using 0.10-m-diameter cores. A subsample of these cores was used to evaluate relative root length and root C concentrations. Root growth in the 0.10–1.0 m depth was measured at 0.10-m depth intervals with a ‘Bartz’ BTC-2 minirhizotron video microscope and I-CAP image capture system (‘minirhizotron’). The video camera was inserted into clear, plastic acrylic minirhizotron tubes (50-mm-diameter) installed within each plot, 30° from the vertical. Root images were captured 4–5 times each season in 2 orientations, left and right side of each tube, adjacent to a furrow, at each time of measurement and the images analysed to estimate selected root growth indices. The indices evaluated were the length and number of live roots at each time of measurement, number of roots which changed length, number and length of roots which died (i.e. disappeared between times of measurement), new roots initiated between times of measurement, and net change in root numbers and length. These measurements were used to derive root C turnover between times of measurements, root C added to soil through intra-seasonal root death, C in roots remaining at end of season, and the sum of the last 2 indices: root C potentially available for addition to soil C stocks. Total seasonal cotton root C potentially available for addition to soil C stocks ranged between ~50 and 400 g/m2 (0.5 and 4 t/ha), with intra-seasonal root death contributing 25–70%. These values are ~10–60% of that contributed by above-ground crop residues. As soil organic carbon in irrigated Vertosols can range between 40 and 60 t/ha, it is unlikely that cotton roots will contribute significantly to soil carbon stocks in irrigated cotton farming systems. Seasonal root C was reduced by cotton monoculture, stress caused by high insect numbers, and sowing Bollgard II varieties; and increased by sowing non-Bollgard II varieties and wheat rotation crops. Permanent beds increased root C but leguminous rotation crops did not. Climatic factors such as cumulative day-degrees and seasonal rainfall were positively related to seasonal root C. Root C turnover was, in general, highest during later vegetative/early reproductive growth. Large variations in root C turnover and seasonal C indices occurred due to a combination of environmental, management and climatic factors.

Nematodes associated with East African Highland cooking bananas and cv. Pisang Awak (Musa spp.) in Central Uganda

A survey was conducted at 17 sites in Central Uganda. Suckers were detached from East African Highland cooking bananas (Musa spp., AAA-group) and the cultivar Pisang Awak (Musa spp., ABB-group), and assessed for nematode population densities and root damage. The frequency of occurrence on both Musa groups was Helicotylenchus multicinctus 88%, Radopholus similis 74% and Pratylenchus goodeyi 50%. Helicotylenchus multicinctus and R. similis densities were higher (P ≤ 0.05) on Highland cooking bananas compared to Pisang Awak, while P. goodeyi densities did not differ significantly between the groups. Helicotylenchus multicinctus and R. similis were observed to be the major pests of Highland cooking bananas in Central Uganda, causing extensive root death. Radopholus similis may be the more important of the two, as it was also highly associated with root necrosis.