Seedling Loss Research Articles

Ravenelia sessilis . [Descriptions of Fungi and Bacteria

Abstract A description is provided for Ravenelia sessilis , which sometimes causes significant loss of seedlings in nurseries. Some information on its dispersal and transmission and conservation status is given, along with details of its geographical distribution (Madagascar, Bangladesh, China (Gansu, Guangdong, Hainan), India (Assam, Bihar, Chhattisgarh, Kerala, Madhya Pradesh, Punjab, Rajasthan, Tamil Nadu, Uttar Pradesh), Myanmar, Nepal, Pakistan, Philippines, Thailand, Australia, Papua New Guinea, Niue and Western Samoa) and hosts (Albizia chinensis, Albizia kalkora, Albizia lebbeck, Albizia lucidior (syn. Albizia lucida ), Albizia odoratissima , Albizia procera and Albizia saponaria ).

Smoke‐Water and a Smoke‐Isolated Butenolide Improve Germination and Seedling Vigour of Eragrostis tef (Zucc.) Trotter under High Temperature and Low Osmotic Potential

AbstractThe seeds of Eragrostis tef are often exposed to high temperatures and low water availability during unfavourable climatic conditions. This affects germination and seedling stand establishment of the crop. Smoke is currently widely studied for its stimulatory role in seed germination. The effect of smoke‐water and a smoke‐isolated butenolide was investigated on E. tef seed germination and seedling growth at different temperatures, light conditions and osmotic potentials. Treating E. tef seeds with smoke‐water and butenolide increased the percentage germination relative to the control at all tested temperatures. These treatments significantly improved seedling length and the vigour index at 25, 30, 35 and 40 and 30/15 °C. Smoke‐water and butenolide increased percentage imbibition from high to low osmotic potential. In comparison with the control, smoke‐water‐ and butenolide‐treated seeds of E. tef showed higher percentage germination with decreasing osmotic potential. Smoke‐water treatment exhibited significantly longer and more vigorous seedlings than the control between 0 and −0.30 MPa osmotic potential. The findings of this study suggest the possibility of using smoke treatments for minimizing E. tef seedling losses at elevated temperatures and water stress.

First Report of White Root Rot of Japanese Serissa Caused by Rosellinia necatrix in Taiwan.

Japanese serissa (Serissa japonica (Thunb.) Thunb.) is a very popular ornamental in Taiwan. During the summer of 2005, serissa plants in a central Taiwan nursery had decaying roots, leaf yellowing, and were wilting. Wilted plants had white fan-like mycelium under the bark. The disease caused 70% loss of seedlings at the nursery. Associated synnemata were rigid, erect, dark, setaceous, 0.8 to 2.1 mm long, 90 to 200 μm wide, and tapering to enlarged whitish gray heads composed of geniculate conidiophores and conidia. Conidia were 3.1 to 5.8 × 2.6 to 3.4 μm, unicellular, hyaline, and subglobal with a truncated base. Root rots were washed, disinfested for 1 min in 0.5% NaOCl, cut into 3 mm3 pieces, plated on Merck (Darmstadt, Germany) potato dextrose agar (PDA) amended with 100 ppm of ampicillin (Sigma, St. Louis, MO), and incubated at 24°C in the dark until hyphae emerged. Single hyphal tips were transferred to PDA, and two isolates were established as pure cultures. Mycelia were cut, stained with 1% cotton blue in lactophenol, and pear-shaped hyphal swellings adjacent to the septa were observed. According to these hyphal, synnematal, and conidial characteristics, the fungus was identified as Dematophora necatrix Hartig, the anamorph of Rosellinia necatrix Prill. Inoculum for pathogenicity tests were produced on oat-wheat medium composed of 20 ml of oat grain and 20 ml of wheat grain, mixed, and soaked in water for 3 h. The grains were placed in 200-ml flasks, autoclaved at 121°C for 30 min, inoculated with two isolates of D. necatrix separately, and grown for 14 days. Six 3-month-old Japanese serissa seedlings were grown in pots. The grain inoculum was added to unsterilized field soil and the plants were transplanted into this mix. Control plants were transplanted into a similar mix without the inoculum. Two replicates were used for a total of 24 inoculated plants and 24 control plants. All plants were kept in a growth chamber at 25 to 35°C with 20 min of irrigation per day, 12 h of irradiation, and relative humidity at more than 95%. Inoculated plants developed root rots after 1 month, and after 4 months, all plants were dead, while control plants remained healthy. D. necatrix was reisolated, hyphal characteristics confirmed, and synnemata were observed on collars of dead plants. The teleomorph was not formed by our cultures, and the identification of Rosellinia necatrix was confirmed by molecular studies. The nuclear ribosomal internal transcribed spacer (ITS) amplified with two primers, ITS1 and ITS4, from our representative isolate demonstrated 99.63, 99.81, and 99.27% similarity to two R. necatrix isolates from Japan and one R. necatrix isolate from Italy, respectively. This disease has been reported on many species of plants (1), but to our knowledge, this is the first report of white root rot of Japanese serissa seedlings caused by R. necatrix in Taiwan. Reference: (1) S. T. Su et al. List of Plant Diseases in Taiwan. The Phytopathological Society of the Republic of China, 2002.

Evaluation of Tomato Culktivar for resistance to wilt disease and their host efficiency to Meloidogyne Incorgnita Infestation

Field and screen house studies were conducted to evaluate the resistance of some tomato cultivars to wilt diseases in Meloidogyne incognita Race 2 naturally infested soil and to test their host efficiency to the nematodes infestation. Five-week old tomato seedlings from each cultivar raised in steam-sterilized soil were transplanted into plots naturally infested with species of plant-parasitic nematodes. The experimental design was a randomized complete block replicated four times. In the screen house trial, five-week old seedlings from each cultivar raised in steam-sterilized soil were transplanted into plastic pots (22.5 cm diameter). Eggs of M. incognita were extracted from pure cultures of the nematodes earlier raised on Celosia argentea L. plant using the chlorox method of extraction. Each seedling was infested with 10,000 eggs of the nematodes. The experiment was laid-out in a completely randomized design with three replications. In the field trials, there were significant incidences of tomato wilt among the tested lines. Three cultivars Tomato-Dalila, Tomato-Akash and Indam-2103 showed tolerance to wilt infection with a range of 9.8 % to 19.7 % seedling loss in 2003 and 2004 respectively. Severities of roots damage caused by nematodes were significantly low in the two field trials. In the screen house evaluation however, none of the cultivars was immune or resistant. The host efficiency test showed that all the tested cultivars were efficient hosts to M. incognita. Growth and yield assessment of the cultivars under field conditions indicated that cultivar F13109 flowered earliest (69.7 days) in 2003 and 72.1 days in 2004 while INDAM-2107 recorded the best luxuriant growth (vigour) in 2004. The highest number of fruit-set per flower cluster was observed in F143 in 2003 and 2004. Cultivar F13019 recorded significantly the highest number of fruits (169.3 and 143.3) while the heaviest weight was observed in AKASH (2.24 kg and 2.7 kg) respectively in both trials. Tomato-Marina recorded statistically the lowest numbers (9.6 and 11.17) and weights (0.23 kg and 0.73 kg) of fruits respectively in both evaluations. Keywords:Meloidogyne incognita Resistance, Tomato, Wilt disease. Nigerian Journal of Horticultural Science Vol. 10 2005: pp. 30-37

RESPONSE OF COWPEA CULTIVARS TO RHIZOCTONIA SOLANI IN FIELD TESTS AT FOUR PLANTING DATES

Rhizoctonia solani is an important pathogen of cowpea (Vigna unguiculata) in the southern U.S. and is the primary cause of seedling diseases in this crop. Stand losses caused by R. solani are especially severe when cowpea is planted in cold, spring soils. Three cowpea cultivars (Coronet, Knuckle Purple Hull, and Mississippi Silver) were evaluated in R. solani-inoculated field tests at four planting dates in Charleston, SC during 2005. The tests were planted on 25 Apr., 9 May, 27 May, and 13 June. The experimental design for each test was a split-plot with six replicates. Whole plots were cultivars, and sub-plots were inoculated with R. solani and an uninoculated control. Rhizoctonia solani caused significant seedling losses in all cultivars planted on the first three planting dates. Seed weight and seed numbers were reduced for `Mississippi Silver' in inoculated plots for all planting dates. In general, `Mississippi Silver' and `Coronet' had higher stand counts and heavier seed yields than `Knuckle Purple Hull', but all three cultivars were susceptible to R. solani. The development of resistant cowpea cultivars would reduce stand losses due to R. solani and improve seed yields of cowpea planted in cold, spring soils.

Relative impact of browsing by red deer on mixed coniferous and broad-leaved seedlings—An enclosure-based experiment

Browsing by mammalian herbivores may help regulate the balance between woody and herbaceous vegetation where agricultural land has been abandoned. Until now, the potential of controlled grazing as a research tool has not yet been fully utilized. In this study, we used an enclosure-based experiment to assess the impact of browsing by red deer on the fate of seedlings on a woodland edge. Species, size, number of lateral shoots, and relative location of seedlings were recorded in 1 m 2 plots ( n = 57) distributed among four vegetation types. From autumn 2000 to winter 2002, after the stay of three tame hinds in the enclosure for 5-day periods in each season (a year-round deer density of 15 hinds/km 2), we checked for loss or browsing damage of seedlings. From autumn 2000 to summer 2001, we additionally recorded the diurnal time spent feeding by the hinds during days 1, 3, and 5, and we simulated their food intake by hand during days 2 and 4. Mortality rates for willow Salix caprea and ash Fraxinus excelsior was higher than for silver fir Abies alba seedlings. Losses due to dehydration, which peaked under coniferous, were counterbalanced by growth of new seedlings. Loss due to browsing was uncommon. Deer impact on coniferous seedlings was insignificant all year long. More broad-leaved seedlings were browsed in spring and summer than in winter. Leader browsing increased with height, most of the taller seedlings occuring in herbaceous vegetation, especially wild cherry Prunus avium. Previously damaged seedlings suffered significantly higher leader browsing than previously undamaged ones. Behavioural observations showed that old pasture was selected by foraging deer all year-round. In autumn, spring and summer, due to quick depletion, the relative contribution of seedlings to the total food intake peaked during day 1, most (>75%) concerning wild cherry. Our results suggest that deer browsing on woody seedlings has more to do with species preference, height structure, growth, and seasonal phenology of seedlings than with induced mortality. Choice of previously damaged seedlings and non-random habitat use by deer can prove useful by minimizing natural regeneration of seedlings close to pastures. Further work is required on long term controlled browsing experiments to better understand natural regeneration of mixed woodland species.

(57) Bioassays and Small-scale Greenhouse Experiments Conducted to Evaluate the Suppression of Phytophthora cinnamomi Activity on Rhododendron × PJM `Elite' by Different Composts Incorporated into Growing Media

Composts vary in their ability to suppress disease activity when incorporated into growing media. Bioassays that enable a reliable and quick assessment of compost's ability to suppress disease activity can save time, funds and space. A bioassay using Lupinu× `Russell Hybrid' seedlings was evaluated as a short-term test for gauging the ability of three composts to suppress activity of P. cinnamomi. Colonized millet seeds were prepared via the V-8 agar method. The colonized millet seed were incorporated into the potting media at 0, 50, 100, 200, and 400 colonized millet seed/200 cc of the media used. Three composts, including composted sewage sludge, brewer's waste and cow manure, were incorporated into the media (50% sand: 50% sphagnum peat, by volume) at rates of 0%, 10%, 25%, and 50%, by volume. The media, including the inoculated millet seed, were placed in small plastic pots (7.6-cm-diameter and 6.7 cm high), after which 10 Lupine seeds were sowed in each pot. Percentage of seedling loss was determined after 43 days of observation. The composted sewage sludge and the cow manure proved suppressive at the 50% incorporation rate and the 10% and 25% rate of the latter compost. The brewer's waste compost proved ineffective in this regard; thus, research with this product was discontinued. In a greenhouse study the same inoculation and compost incorporation rates were used, but rooted cuttings of Rhododendro × PJM `Elite' were plotted into the various treatments. Suppression of disease activity by the composts was significant 2 and 4 months after initiation of treatments. Significance in disease suppression noted between these treatments decreased significantly during the fifth month of the experiment.

RESPONSE OF COWPEA CULTIVARS TO RHIZOCTONIA SOLANI IN FIELD TESTS.

Rhizoctonia solani is an important pathogen of cowpea (Vigna unguiculata) in the southern U.S. and worldwide. Cowpeas are especially susceptible to seedling diseases caused by R. solani when planted in cold, moist, spring soils. Nine cowpea cultivars were evaluated in inoculated field tests at six planting dates in Charleston, S.C., during 2004. The cowpea cultivars evaluated were Bettergro Blackeye, Knuckle Purple Hull, Mississippi Silver, Colossus-80, Charleston Nemagreen, Texas Cream-40, White Acre, Coronet, and Charleston Greenpack. The tests were planted on 20 Apr., 29 Apr., 11 May, 19 May, 27 May, and 8 June. The experimental design for each test was a split-plot with six replicates. Whole plots were cultivars, and sub-plots were inoculation with R. solani and an uninoculated control. Rhizoctonia solani caused significant seedling losses in all cultivars evaluated during mid-April to early June and seed yields were reduced in the 11 May planting. In general, standard cowpea cultivars (Mississippi Silver, Colossus-80, and Coronet) had higher stand counts and produced heavier seed yields than other cowpea cultivars, although these standard cultivars were not resistant to R. solani. Resistant cowpea cultivars are needed to allow earlier planting of the crop in cold soils, which would extend the growing season and allow more efficient use of harvesting equipment and processing facilities.

Performance of novel insecticide seed treatments for managing onion maggot (Diptera: Anthomyiidae) in onion fields

Management of onion maggot, Delia antiqua (Meigen), in onion requires the use of an insecticide applied at planting. Insecticide resistance and a dearth of available products have stimulated an effort to identify new insecticides for onion maggot control, especially chemistries that can be delivered as seed treatments. Onion seeds film-coated with fipronil, spinosad, clothianidin or thiamethoxam were evaluated for onion maggot control in muck soils located in two major onion-growing regions in New York, USA in 2001–2003. Fipronil, spinosad and clothianidin at rates of 25, 25 and 50 g (a.i.)/kg of seed, respectively, consistently provided excellent control of onion maggot (⩽5% seedling loss). Moreover, these seed treatments performed well during wet and dry seasons. In contrast, thiamethoxam at a rate of 50 g (a.i.)/kg of seed failed to control onion maggot in three of the five experiments. Cyromazine seed treatment, which is currently used by onion growers, provided an equivalent level of onion maggot control to fipronil, spinosad and clothianidin. The other commonly used treatment, chlorpyrifos in-furrow drench, performed poorly in three of the four experiments, resulting in an average seedling loss of 17%. Insecticide resistance was implicated as the reason for the failure in two of the trials where seedling loss averaged 24%. Seed treatments provide growers with a convenient, reliable and environmentally responsible method for managing economically damaging onion maggot infestations, including those that are resistant to chlorpyrifos. Fipronil, spinosad and clothianidin seed treatments should be considered for future registration on onion. Availability of these chemistries would allow rotation with the cyromazine seed treatment, which would likely prolong the longevity of all products.

An Outbreak of Bacterial Speck Caused by Pseudomonas syringae pv. tomato on Tomato Transplants Grown in Commercial Seedling Companies Located in the Western Mediterranean Region of Turkey

A serious outbreak of a leaf spot disease was observed on tomato transplants grown in commercial seedling companies in southwestern Turkey (Antalya) during the springs of 2002 and 2003. Disease incidence was more severe in the western Mediterranean Region of Turkey. Occurrence of the outbreak resulted in approximately 20 and 25% seedling losses in the springs of 2002 and 2003, respectively. The initial symptoms consisted of pronounced water-soaked, dark brown-to-black spots on young expanding leaves that were 1 to 2 mm in diameter. Later, a number of leaf spots on older leaves enlarged and coalesced, causing leaf desiccation and finally, seedling death. In addition, in 2003 the disease incidence was approximately 5% in 142 commercial greenhouses. Tomato production was unaffected since significant outbreaks did not occur on greenhouse plants. No fruit symptoms were observed. Twenty-six strains were isolated from diseased tomato seedlings and plants from different greenhouses located in different places and all were gram negative and fluorescent on King's B medium. All strains were levan and gelatin liquefaction positive and oxidase and arginine dihydrolase negative. None of the 26 strains utilized erythritol and l-lactate as the sole carbon source (1,2). Fatty acid analysis identified the strains as Pseudomonas syringae pv. tomato with similarity indices ranging from 0.876 to 0.932%. Pathogenicity of the isolates was confirmed on 4-week-old tomato seedlings (cv. Biotek Selin) sprayed with the bacterial suspensions containing 108 CFU/ml of sterile water. Later, a number of leaf spots on the leaves enlarged and coalesced, causing leaf desiccation. Inoculated and control tomato seedlings were covered with polyethylene bags and placed in a growth chamber at 25°C for 48 h and then the bags were removed. Small (1 to 2 mm), water-soaked, dark brown-to-black spots similar to those observed in the greenhouses of commercial seedling companies and commercial greenhouses that produce tomato developed on the young expanding leaves of inoculated plants within 7 to 10 days. No symptoms developed on control plants. The bacterium was reisolated from inoculated plants and identified as a strain of Pseudomonas syringae pv. tomato. Koch's postulates were fulfilled. To our knowledge, this is the first report for the occurrence and outbreak of the bacterial speck disease on tomato transplants in greenhouses of commercial tomato seedling production companies in Turkey.

Seedling Reproduction Established with Soil Scarification within an Oak Overwood after Overstory Removal

Abstract We report on the density and height of seedling reproduction established with and without soil scarification in a mixed-oak two-stage shelterwood after overstory removal. In 1993, shallow soil scarification was conducted to bury recently disseminated northern red oak (Quercus rubra L.) acorns collected from another site and disrupt red maple (Acer rubrum L.) seedling reproduction that dominated the understory. The shelterwood overstory was removed in the winter of 1997/1998. The following spring, seedling density decreased in response to harvest activities. However, one growing season after overstory removal, seedling density for most species or species groups had recovered to preharvest levels. At that time (5 years after soil scarification), scarified plots had 16 times more northern red oak (35,119 ha−1), 50% fewer (8,076 ha−1) red maple, and 37% more total (55,432 ha−1) seedlings compared to undisturbed control plots. Other species of oak, which generally lacked mast at the time of scarification, had significantly reduced density (59% less, at 4,405 ha−1), suggesting that scarification treatments should coincide with viable mast or net loss of oak seedlings may result. Five growing seasons after treatment, the greatest proportions of seedlings were in the tallest two height classes (33–122 cm and >122 cm) and were northern red oak. In summary, 5 years after incorporating viable mast into the soil with shallow scarification within a mixed-oak shelterwood and 1 year after overstory removal, scarified areas had more advance reproduction dominated by large and vigorously growing oak seedlings, while undisturbed areas had lower seedling density of primarily large red maple.

Pathogenic variability of rice blast isolates in Nepal

Blast, caused by Pyricularia grisea Sacc., is a major disease of rice (Oryza sativa L.) in Nepal. It causes seedling loss (Chaudhary and Sah 1998) and adversely affects plant growth and yield (Chaudhary 1999). The fungus is highly variable, making disease management complicated. This study was undertaken to understand the variability of blast isolates in order to deploy resistance genes properly. Diseased leaves were collected on diverse genotypes from different locations. Twenty-five monoconidial isolates were obtained and maintained in agar slants during 2000 following the procedures of Bonman et al (1986). Isolates were multiplied on prune agar plates and inoculated at 105 mL-1 on 3-5-leaf stage seedlings of 10 differential varieties in a greenhouse in 2001 (Table 1). The 10 seed- lings of each differential, replicated thrice, were raised in aluminum trays, filled with farmyard manure, and fertilized with 150-22-0 kg NPK ha-1.

Influence of initial root length on physiological responses of cherrybark oak and Shumard oak seedlings to field drought conditions

One-year-old bareroot seedlings of cherrybark oak ( Quercus pagoda Raf.) and Shumard oak ( Quercus shumardii Buckley) were studied to describe their sensitivity to changes in environmental moisture in relation to initial root characteristics at the time of planting. Shumard oak seedlings with greater initial lateral root lengths and shoot heights sustained higher leaf gas exchange rates during both non-drought and drought periods. On the contrary, cherrybark oak seedlings had high leaf water potential ( Ψ) and water use efficiency (WUE) and maintained low stomatal conductance ( g s). This strategy generally allows for adequate gas exchange while minimizing water loss during drought. Initial lateral root:shoot ratio (LRSR) was negatively related to leaf gas exchange and water status, suggesting that excessive water loss of large seedlings with large root volume may negatively affect their drought hardiness. The responses of these floodplain oak species to drought was likely mediated by the absolute development of root systems at the time of planting and the balance between functioning root absorbing surface and transpirational leaf area. Responses of both species to drought in the subsequent growing seasons must be studied to determine their unique responses to water stress.

Effects ofRhododendron maximumThickets on Tree Seed Dispersal, Seedling Morphology, and Survivorship

In the southern Appalachian forests, the regeneration of canopy trees is severely inhibited by Rhododendron maximum L., an evergreen understory shrub producing dense thickets. While light availability is a major cause, other factors may also contribute to the absence of tree seedlings under R. maximum. We examined the effects of R. maximum on several life history stages of tree species, including seed dispersal, seed bank germination, seedling growth, and survivorship. We found no significant effect of R. maximum on seed reaching the forest floor for Acer rubrum, Liriodendron tulipifera, Quercus rubra, Quercus prinus, Carya spp., and Nyssa sylvatica. This indicates that either seed output of maternal trees rooted within the thicket were unaffected by R. maximum or seed dispersal from surrounding areas into thickets compensated for a lower seed production of canopy trees rooted in the thickets. Germination of tree seeds (A. rubrum, L. tulipifera, Q. rubra, and Betula lenta) from the seed bank also was not reduced by leaves and substrates within the thickets. Seedling mortality of all species (Q. rubra, Prunus serotina, and Tsuga canadensis) planted in our experimental plots was up to fivefold higher in thickets of R. maximum compared with those outside the thickets. The order of mortality under the R. maximum thickets, $$Prunus> Quercus> Tsuga$$, was consistent with the shade tolerance ranking of these species. Loss of Tsuga seedlings was attributed to burial by litter rather than shade. Surviving seedlings of Quercus and Prunus in R. maximum thickets were taller than those outside the thickets, but the seedlings in R. maximum thickets produced significantly fewer leaves, smaller total leaf area, leaf mass, and stem mass. Leaf N (%) was significantly higher in Quercus seedlings in R. maximum thickets compared with seedlings outside the thickets. Moreover, no difference was found in leaf N (%) between forest types for Prunus and Tsuga, indicating that seedlings in R. maximum thickets were not N limited. Rather, light limitation, herbivory, and litter fall contributed to the lack of tree regeneration under R. maximum thickets.

Wildfire alters oak growth, foliar chemistry, and herbivory

Fire is a major disturbance factor influencing the formation and maintenance of oak ( Quercus)-dominated forests of eastern North America. Fire suppression practices have resulted in declining oak dominance, and caused the loss of oak seedlings due to poor light regimes and vegetative competition. Both wildfires and prescribed fires can alter forest stand composition and influence succession. Fire can also cause physiological changes in plants, potentially influencing growth, competition, and susceptibility to herbivory. Fire may impact herbivore populations directly by altering habitat and abundance, or indirectly through alterations in food supply. The objective of my study was to assess the effects of a naturally occurring wildfire on chestnut oak ( Q. prinus L.) seedling growth, foliar chemistry, and suitability to a generalist herbivore, the gypsy moth ( Lymantria dispar L.). I sampled chestnut oak seedlings four times for phytochemical analysis throughout the post-fire growing season, and sampled once to assess suitability for caterpillar performance and to measure seedling growth. Chestnut oak seedlings sampled from burned sites had significantly higher foliar nitrogen and water content than seedlings sampled from non-burned sites. Seedlings from burned sites had transient declines in foliar carbohydrate concentrations, and also had higher initial tannin levels. Although seedlings from burned sites were smaller with respect to stem diameter and height, they grew more with respect to absolute growth rate, leaf area, and biomass, than their non-burned counterparts. In spite of the differences in leaf chemistry, there were no significant differences in the growth or development of gypsy moth caterpillars fed foliage from burned versus non-burned seedlings.

First Report of Fusarium oxysporum f. sp. lentis of Lentil in Italy.

During June and July 1999-2000, a disease causing severe losses of seedlings of a local ecotype of lentil (Lens culinaria Medik, syn. Lens esculenta Moench.) was observed in four commercial fields in central Italy (Umbria and Latium regions). Disease symptoms, observed in both years, included stunting, wilting, marked reduction of root system, and internal vascular discoloration of the lower stem. Twenty-five samples for each field, consisting of lentil roots and stems, were collected. Seven to eight tissue pieces per sample, derived from the lower stem and a variety of root sizes, were sampled, surface-treated in 1 or 0.1% HgCl2 for 30 s, rinsed in sterile deionized water, placed on petri dishes containing potato dextrose agar (PDA, pH 5.5) and then incubated for 7 days at 25°C. F. oxysporum (2) (identification confirmed by R. A. Samson, Centraalbureau Voor Schimmelcultures of Baarn, Netherlands) was consistently recovered from affected tissues. Monoconidial cultures of four isolates were prepared and evaluated for pathogenicity. Seventy 4-week-old lentil seedlings (local ecotype) were inoculated by dipping the root system of each seedling in a spore suspension of each isolate of F. oxysporum (106 conidia/ml) for 30 min and transplanted into a pot containing a sterilized sandy-loam soil mix. In a second experiment, 100 seeds for each isolate were sown in the same soil mix amended with rice kernels colonized with four isolates of F. oxysporum (10% wt/wt). Seedlings dipped in sterile deionized water and seeds sown in soil amended with noncolonized rice kernels served as controls in the first and second experiments, respectively. Three replications of both pathogenicity tests were carried out in a controlled growth chamber at day/night 22/20 ± 2°C, 60/70% RH, 12 h day (180 μE•m2•s-1). Both experiments were repeated. Two to three weeks after inoculation, all plants inoculated with each F. oxysporum isolate showed the same symptoms observed in the field and caused wilting and death of 80 to 100% of the inoculated plants. F. oxysporum was consistently reisolated from the symptomatic test plants, whereas the fungus was never isolated from all control plants which remained symptomless and healthy. F. oxysporum was not observed in seed health tests (blotter and agar plate methods) carried out on 100 seeds per sample harvested from infected crops. Management practices include long-term crop rotation with nonsusceptible hosts and removal and destruction of infected crop debris. Seed treatment with fungicides (benzimidazoles, thiram) (3) can reduce incidence of the disease but host resistance is the best mean of controlling Fusarium wilt. Most resistant lentil accessions come from Chile, Egypt, India, Iran, and Romania (1). This is the first report of F. oxysporum f. sp. lentis on lentil in Italy. Without adequate control, this seedling disease may become a major factor limiting production of high quality lentils in Umbria and Latium.

Low density of prickly acacia under sheep grazing in Queensland

Populations of an introduced woody weed, prickly acacia (Acacia nilotica (L.) Delile ssp. indica (Benth.) Brenan syn. Acacia arabica (Lam.) Willd. ssp. indica Benth.), were surveyed at 4 sites in central Queensland. There is a significantly lower frequency of plants of 3 m in height within populations which have been grazed by sheep, indicating that browsing by sheep reduces regeneration. There were higher losses of seedlings at a sheep-grazed site than at cattle-grazed sites. These results support previous assertions that prickly acacia is regenerating more successfully on cattle properties, because cattle both disperse seeds and are less effective herbivores. In regions of low annual rainfall, prickly acacia is capable of forming dense stands (up to 2,700 shrubs ha(-1)) in lowland landscape types. Stands are less dense in upland landscapes (maximum of 718 shrubs ha(-1)). Of most concern is that in regions of high annual rainfall prickly acacia can form extremely dense thickets across most landscape types (up to 3,400 shrubs ha(-1)). We suggest that prickly acacia is most likely to become a management problem on cattle properties, and an extreme problem in high annual rainfall areas. The inclusion of sheep in livestock rotations may be an effective control measure in the Mitchell Grasslands, but this may not always be possible. A high priority is to prevent prickly acacia from expanding its range into equivalent high rainfall areas within Queensland, and also in the Northern Territory, northern New South Wales, and Western Australia. This could be achieved by quarantining livestock which have come from infested properties until seeds have passed through the digestive tract, after about 6 days. Management strategies at the property level should aim to prevent further spread of prickly acacia by controlling cattle movements between paddocks during periods when cattle are ingesting pods and seeds.

231 Effects of Stand Deficiencies and Timing of Replanting on Yields of `Athena' Muskmelon

Seedling losses shortly after emergence in muskmelon (Cucumis melo L.) can be potentially devastating to growers. Muskmelon growers often have problems with obtaining adequate stands and need to understand the affects of replanting seed into poor stands. Field studies were conducted over 2 years to determine if replanting (at 1, 2, 3, or 4 weeks after the initial seeding) into stand deficiencies of 10%, 30%, and 50% affected `Athena' muskmelon size and yield. `Athena' muskmelon stand deficiencies up to 30% does not appear to reduce total or marketable numbers, but stand deficiencies of 50% or more will decrease total and marketable melon yields. Replanting into 10%, 30%, and 50% stand deficiencies will increase early season melon numbers regardless of the replant times used. For main-season and total-season harvests, there was no advantage of replanting into 10% deficient stands, and in most cases, replanting reduced total and marketable melon numbers. In the 1997 experiment, replanting into 30% and 50% stand deficiencies improved yields but this did not occur in the 1998 experiment. `Athena' muskmelon should be replanted only if a stand reduction of ≈50% or more occurs. Melon numbers were generally higher if replanted by 1 or 2 weeks compared to 3 or 4 weeks, but the timing of replanting does not appear to have significant influence on total or marketable melon numbers.

Characterization, establishment and persistence under grazing of nitrous oxide‐induced octoploid Phalaris aquatica L.

AbstractThe increased seed size of induced polyploid forms has been an important component of improved seedling vigour of small‐seeded pasture species. Nitrous oxide was used to produce an octoploid form of the F3 generation of a cross between the closely related tetraploid cultivars, Uneta and Australian phalaris (Phalaris aquatica L.; 2n=28). A high degree of recovery of octoploid plants (30%) was attained by nitrous oxide treatment. Increases in seed weight, glume length, stomatal guard cell length and pollen grain diameter of 13–84% were recorded in glasshouse‐grown plants of the octoploid compared with the tetraploid parent population. In a field experiment the superior establishment of the octoploid compared with the tetraploid was interpreted as evidence of the better establishment capacity of the octoploid from deep placement of the seed. However, there was a greater loss of octoploid seedlings over summer, and a greater reduction in the basal area of the sward under continuous grazing than in the tetraploid. Although the octoploid swards produced significantly less phalaris dry weight than the tetraploid in exclosures, the total herbage dry weight of the two types was not significantly different in three of four cuts taken over 2 years. This result supports the view that the more erect, sparsely tillered growth habit of the octoploid phalaris is conducive to the productivity of other sward components.

Aphanomyces euteiches Inoculum Potential: A Rolled-Towel Bioassay Suitable for Fine-Textured Soils.

A sensitive measure of soil inoculum potential is needed to evaluate field management of common root rot (Aphanomyces euteiches) in peas (Pisum sativum). A modified rolled-towel (RT) bioassay had been proposed to measure soil inoculum potential in fine-textured soils used for pea production in Minnesota. Homogenized soil was used because organic debris containing the inoculum could not be separated by wet sieving. The poor precision prompted an evaluation of procedures to improve this modified RT bioassay. Seed treatment with a 5% solution of sodium hypochlorite before pea seed germination and plant isolation procedures during the RT bioassay preparation/incubation reduced seedborne contamination and seedling loss to less than 5%. Tests conducted with pasteurized soil that was artificially infested with oospores showed the region of the pea taproot 1 to 2 cm below the seed to be more susceptible to infection (33% compared with 15% infected seedlings) than the region 1 to 2 cm above the root tip. A soil volume of 1.0 cm3 increased inoculum potential compared with 0.5 cm3 applied to each seedling but did not influence the random error; the 40-seedling compared with the 20-seedling RT bioassay reduced random error from 18 to 12%. The modified RT bioassay conducted on soil that was artificially infested after steam treatment or without steam treatment showed superior performance when using 40 seedlings compared with 20 seedlings when evaluated for accuracy and precision. Multiple infection theory demonstrated more multiple infections in the RT bioassay with a 0.5 cm3 soil volume applied to each seedling, which shows that soil mass is a factor preventing a higher percentage of infected seedlings. These modifications to the RT bioassay improved the method enough to reduce the random error by one-half compared with using homogenized soil without the proposed modifications.