Functional significance of triazine-herbicide resistance in defence of Senecio vulgaris against a rust fungus

We investigated the functional significance of plant performance (dry mass, photosynthesis) in plant defence (resistance and tolerance) against pathogen infection, and potential negative cross-resistance between herbicide resistance and plant defence against disease. We compared isonuclear triazine-herbicide-resistant (TR) and -susceptible (TS) biotypes of Senecio vulgaris, in the presence and absence of infection by the rust Puccinia lagenophorae. In a growth chamber study with two reduced irradiance levels, rust infection had a severe effect on plant performance with infected plants having 55% less dry mass and 54% reduced whole-plant photosynthesis than non-infected plants. The TR biotype was more susceptible (reduced resistance) to the pathogen, but the biotypes did not differ in their ability to compensate for rust infection (tolerance). TR plants were less productive than TS plants when grown non-shaded (ca. 10% full sunlight) but not when shaded (ca. 5% full sunlight). This is especially important for situations, where S. vulgaris grows under the crop canopy (e.g. in maize). Here, very low light levels might contribute to a numerical increase of TR relative to TS plants even when only occasionally treated with triazine. Whole-plant photosynthesis was reduced by 21% in TR plants as compared to the TS biotype, and by 59% in plants grown in the shaded as compared to the non-shaded treatment. When whole-plant photosynthesis values were corrected for the estimated leaf area of plants, we found no significant variation between biotypes, shade treatments or rust treatments. In experimental mixed TR:TS field populations, the proportion of TR plants decreased more rapidly in rust-infected populations than uninfected. This finding, together with the lower resistance in the TR than the TS biotype to the rust fungus observed in the growth chamber experiment, may indicate negative cross-resistance, which is a potential tool in the management of herbicide-resistant weeds.     

Biology, Impact and Preliminary Host-specificity Testing of the Rust Fungus, Uromyces salsolae , a Potential Biological Control Agent for Salsola kali in the USA

Russian thistle, Salsola kali (=Salsola australis) (Chenopodiaceae) of Eurasian origin is a troublesome weed in the drier regions of the western USA. The weed commonly infests crops, disturbed wastelands and overgrazed rangelands. A rust fungus, Uromyces salsolae , has been found to cause considerable damage to the weed in Eurasia. Field observations in Turkey revealed that S. kali plants infected by the rust were covered with a powdery brown mass of urediniospores and had stunted growth. Under laboratory conditions, U. salsolae severely infected Salsola plants from France, the USA and Turkey (control). The urediniospores germinated when in contact with water within a minimum period of 2 h and over a wide range of temperatures. S. kali plants were also infected at a wide range of temperatures and incubation periods with simulated dew. Fungal attack was very severe and caused mortality or much reduced growth of infected plants without seed production. Preliminary host specificity testing using 17 plant species or varieties from six families revealed that the rust infected only the Russian thistle. U. salsolae was imported recently into the USA for further host specificity testing under quarantine conditions. Its use as a biological control agent for S. kali in the USA is recommended.     

Evaluation of Puccinia carduorum for biological control of Carduus pycnocephalus in Tunisia

Puccinia carduorum, a rust fungus from Italian thistle in Tunisia, was most aggressive on young growth stages of the weed in greenhouse tests. Repeated inoculations with the fungus significantly reduced weed biomass. Host-range tests suggest the fungus may be a safe biological control agent of Italian thistle in Tunisia.     

Effects of Selected Herbicides on the Germination and Infection Process of Puccinia lagenophora,a Biocontrol Pathogen of Senecio vulgaris

The herbicides 2,4-D, glyphosate, linuron, and MCPP at rates of 1X (recommended field rate), 0.25X, 0.025X, and 0.0125X were evaluated in vitro for their effects on the rust fungus Puccinia lagenophorae, a biocontrol agent for the annual weed Senecio vulgaris. Herbicides applied at 1X and 0.25X completely prevented aeciospore germination. Glyphosate was toxic even at 0.0125X and 0.025X. Aeciospores germinated in linuron, 2,4-D, and MCPP at 0.025X and 0.0125X at rates similar to the water control. Abnormal germ-tube growth was observed with 2,4-D at 0.25X and 0.025X, with linuron at 0.025X, and with glyphosate at 0.0125X. Further in planta studies were perfomed with two inbred lines of S. vulgaris inoculated with aeciospores of P. lagenophorae and treated with water, linuron, and 2,4-D at 0.025X at different times of application. Quantitative analysis of the infection process revealed that both herbicides reduced spore deposition on the leaves and altered leaf morphology. The herbicides had no effect on disease severity at this low rate although linuron significantly reduced the formation of infection peg. Timing of herbicide application had no influence on the infection process, and the effect of the herbicides on fungal development did not differ between the two plant lines. Thus, the herbicides applied at 0.025X did not increase plant susceptibility to the rust fungus, and the rates of 1X, 0.25X, and 0.025X would prevent, inhibit, or delay fungal development. Therefore, joint application of P. lagenophorae with these herbicides to control S. vulgaris cannot be recommended.     

The Effects of Nutrient Deficiency and Rust Infection on the Relationship Between Root Dry Weight and Length in Groundsel (Senecio vulgaris L.)

Groundsel (Senecio vulgaris L.) was grown in sand culture ata range of nutrient concentrations. Except when nutrient deficiencywas severe, infection by the rust fungus Puccinia lagenophoraeCooke substantially reduced root dry weight but had little effecton root length. Thus, specific root length (SRL, cm root mg^–1d. wt) was significantly increased in rust-infected plants.The inhibition of root dry weight caused by rust infection wasmost pronounced late in development, especially after floweringwhen, in control plants, root elongation but not dry weightaccumulation ceased. In rusted plants, and in all plants subjectedto severe nutrient deficiency, dry weight accumulation in theroots ceased concurrently with root elongation. Late in developmentat high nutrient concentration adventitious roots with low SRLswere produced. However, infection did not modify the productionof such roots and increases in SRL could not be attributed tochanges in any single type of root. There was an inverse relationship between SRL and root diameter.This relationship was unaffected by rust infection whilst nutrientdeficiency changed only its intercept: at a given SRL rootsof nutrient stressed plants were thinner than those of plantswith adequate nutrient supply. Thus, the smaller diameter ofroots of nutrient-stressed plants occurred independently ofmeasured changes in SRL but, in the absence of nutrient stress,the decrease in root diameter caused by rust was closely relatedto increases in SRL. Changes in the root: length relationships in rusted plants mayhave important implications for root activity in the field.In view of the reported changes in SRL, inhibition of root growthin terms of dry weight may be a poor indicator of potentialchanges in activity. Senecio vulgaris, rust infection, nutrient deficiency, root weight: length ratio, root diameter     

Low genetic differentiation among seasonal cohorts in Senecio vulgaris as revealed by amplified fragment length polymorphism analysis

Common groundsel, Senecio vulgaris (Asteraceae), is a highly selfing semelparous ephemeral weed that belongs to the few plant species in central Europe capable of growing, flowering and fruiting all year round. In temperate climates, flowering S. vulgaris cohorts were found to appear up to three times per year. Using amplified fragment length polymorphism (AFLP) molecular markers we examined temporal genetic differentiation among spring, summer and autumn cohorts at each of seven sites located in two regions in Switzerland. Strong genetic differentiation among cohorts may indicate the existence of seasonal races of S. vulgaris, reproductively isolated by nonoverlapping flowering phenologies. Analysis of molecular variance (amova) revealed that < 2.5% of the AFLP variation resided among cohorts within sites, whereas there was significant genetic differentiation among plants from different sites (15.6%) and among individuals within cohorts (81.9%). Significant genetic differentiation was also observed between the two regions. Isolation-by-distance was found on a regional scale, but not on a local scale. Gene flow was estimated to be approximately 15-fold higher among cohorts within sites than among sites. We further found, on average, similar levels of genetic diversity within the three seasonal cohorts. The results of this study demonstrate that season of growth represents a weak barrier for genetic exchange among S. vulgaris populations and does not affect molecular variance. Therefore, there is no evidence for the existence of seasonally specialized races of S. vulgaris. We discuss some implications of the results for the biological control of S. vulgaris using a native rust fungus.     

Fungicidal activity of Ranunculus asiaticus and other weeds against Fusarium oxysporum f.sp. lycopersici

The effects of aqueous extracts of some common weed species against Fusarium oxysporum Schlecht f.sp. lycopersici (the causal agent of tomato wilt disease) were investigated under laboratory conditions. Anagallis foemina L., Cerastium dicotomum L., Falcaria vulgaris L., Ranunculus asiaticus L., Scorpiurus mur-icatus L. and Solanum nigrum L. extracts were the most toxic to the fungus. Further studies on buttercup (Ranunculus asiaticus L.) showed that fresh shoot extract of this species prevented growth of F. oxysporum when incorporated into agar medium. Extracts of different parts of the plant inhibited fungus growth and sporulation, but the fungitoxicity decreased with incubation period with only slight changes in the toxicity of fresh shoot extract. The shoot and fresh parts extracts were more toxic than root and dried tissue extracts. Addition of 0.5 ml fresh shoot or 1 ml fresh root extract to the growing medium significantly reduced fungal colony growth, and the effect was extract concentration dependent. Fresh shoot extract of R. asiaticus added to a liquid medium significantly reduced mycelial dry weight compared with the control, and incorporation of 0.1 g dried shoot or 0.2 g dried roots in the media strongly inhibited fungus growth. Results of a pot experiment showed no harmful effects of R. asiaticus extracts on tomato growth.     

PR-1 protein inhibits the differentiation of rust infection hyphae in leaves of acquired resistant broad bean

Treatment of broad bean leaves with salicylic acid (SA) or 2, 6-dichloro-isonicotinic acid (DCINA) induces resistance against the rust fungus Uromyces fabae resulting in reduced rust pustule density. Light-microscopy studies showed that in induced resistant plants the rust fungus is inhibited immediately after penetration through the stomatal pore. The differentiation of infection structures growing within the intercellular space of the leaf, i.e. infection hyphae and haustorial mother cells, is inhibited. Furthermore, low-temperature scanning electron microscopy studies of freeze fractures revealed protrusions at the tips of infection hyphae growing in induced resistant broad bean leaves. Treatment of in vitro-differentiating rust infection structures with intercellular fluids (IFs) from induced resistant plants confirmed that the fungus is sensitive towards an apoplastic anti-fungal activity only after having formed appressoria. Other legume rusts such as U. vignae and U. appendiculatus were likewise inhibited in the presence of IF from SA-treated broad bean leaves. Heterologous antibodies were used to study changes in the extracellular pathogenesis-related (PR) protein pattern after resistance induction. Western blots indicated that chitinases and beta-1,3-glucanases were present in both induced and control plants. In contrast, PR-1 proteins were newly synthesized in response to SA or DCINA application. The major induced PR-1 protein was purified and exhibited strong differentiation-inhibiting activity towards U. fabae infection structures. We conclude that the inhibition of rust infection hyphae in acquired resistant broad bean plants is mainly due to the anti-fungal activity of this induced basic PR-1 protein.     

Host-specificity of Uromyces heliotropii, a fungal agent for the biological control of common heliotrope (Heliotropium europaeum) in Australia

Common heliotrope, Heliotropium europaeum (Boraginaceae), a summer annual, is a serious weed of pastures in Australia causing poisoning in sheep and other grazing animals. The weed is native to Mediterranean and Middle Eastern regions of Eurasia where it is attacked by several arthropods and pathogens including the rust fungus Uromyces heliotropii. The host-specificity of a virulent and effective strain of the rust from Turkey has been tested by inoculating 96 plants of importance to the Australasian region using both microscopic and macroscopic observations of the reaction of host and non-host plants. The test plants included several members of the Boraginaceae and related families as well as species of Heliotropium native to Australia. The infection was mostly limited to European species of Heliotropium. U. heliotropii was thus found to represent a safe introduction and has recently been introduced to Australia for the biological control of common heliotrope.     

Modeling the impact of a biocontrol agent, Puccinia lagenophorae, on interactions between a crop, Daucus carota, and a weed, Senecio vulgaris

The impact of a biocontrol agent spreading from a point source on crop–weed interactions was modeled. The model encompassed: (i) severity of crop–weed competition as affected by a rust pathogen, (ii) velocity of spread of the rust pathogen, and (iii) density of infected plants introduced in the weed population as starting points (inoculum sources) for an epidemic. The model was parameterized for a study system encompassing the crop Daucus carota (carrot), the weed Senecio vulgaris (common groundsel), and its antagonist Puccinia lagenophorae. The parameters of (i) were estimated in a greenhouse study using a response-surface design. Estimates of the parameter of (ii) were obtained from the literature. The density of infected plants (iii) was varied to simulate crop loss as function of density. Simulations were run for various densities of the weed and various velocities of rust pathogen spread. The results of the simulations indicated a crop-loss ranging from 5 to 10% at levels of relatively weak D. carota–S. vulgaris competition. Density of inoculum sources and velocity of P. lagenophorae spread had only minor effects on crop loss. In contrast, density of inoculum sources and velocity of spread had major effects on crop loss at levels of intermediate (range of 10–35% loss) and severe competition (range of 30–70% loss). The results are discussed both with respect to biological control of S. vulgaris using P. lagenophorae as biocontrol agent and as a general model describing the impact of the spatial dynamics of a pathogen (natural enemy) on plant competition.     

Chitinases of <Emphasis Type="Italic">Coffea arabica</Emphasis> genotypes resistant to orange rust <Emphasis Type="Italic">Hemileia vastatrix</Emphasis>

Two Coffea arabica — Hemileia vastatrix incompatible interactions (I₁: coffee cv. Caturra — rust race VI and I₂: coffee cv S4 Agaro — rust race II) and a compatible interaction (coffee cv. Caturra — rust race II) were compared in relation to the infection process and chitinase activity. In the two incompatible interactions the fungus ceased growth in the early infection stages, while in the compatible interaction no fungus growth inhibition was observed. A high constitutive level of chitinase activity was detected in the intercellular fluid of healthy leaves. Upon infection, chitinase isoforms were more abundant in incompatible interactions than in the compatible interaction. Immunodetection showed that class I chitinases are particularly relevant in the incompatible interactions and might participate in the defence response of the coffee plants.     

Complementing cultural weed control with plant allelopathy: Implications for improved weed management in wheat crop

Wheat (Triticum aestivum L.) is an important cereal crop in Pakistan which is suffering from major grain production loss because of weed infestations. Control of weeds by herbicides is a primary weed management tool in wheat crop which can be detrimental to the environment and grain produce. Development of an efficient and eco-friendly alternate to the herbicidal weed control, testing the effectiveness of cultural weed control (crop row orientation, selected wheat genotypes and hand weeding) and plants water extracts was undertaken for weed control in wheat. An experiment was run under field conditions in winter season in 2016–2017 and in 2017–2018 in Khyber Pakhtunkhwa Province, Pakistan. The repeated experiment was each time undertaken using a randomized complete block design with a double split plot arrangements at the New Developmental Farm, University of Agriculture, Peshawar, Pakistan. The crop row orientations used were assigned to the main plots. The wheat genotypes used were assigned to the sub-plots. The allelopathic water extracts and hand weeding were assigned to the sub-sub plots. The averaged mean values for row orientations of both the years revealed lowest weed density (95.7 weeds m^?2), highest grains per spike (47.3), 1000 grains weight (44.7 g) in north to south row orientation. The averaged mean values of weed density (101.6 weeds m^?2), grains per spike (48.2), 1000 grains weight (45.9 g), crude protein content (12.793%), crude fat content (1.533%) and ash content (1.586%) were greater for the wheat genotype Pirsabaq-2013 and Atta-Habib-2010. Water extract of S. halepense, P. hysterophorus, H. annuus and hand weeding showed significantly lower weed density (84.0 to 93.3), grains per spike (50.9 to 48.3), 1000 grains weight (48.3 to 46.2 g), grains protein content (12.280 to 12.209%), grains crude fat content (1.471 to 1.464%) and grains ash content (1.523 to 1.515%). Interaction effect of different tested weed control treatments i.e. N-S × Pirsabaq-2013 and Atta-Habib-2010 × water extract of S. halepense, P. hysterophorus and H. annuus were found to show further reduction in weed density and enhance grains per spike and grains nutrition contributing parameters. Our results show that sowing wheat genotypes Pirsabaq-2013 and Atta-Habib-2010 in north-to-south row orientation, and application of water extract of S. halepense, P. hysterophorus, H. annuus can give an effective weed management and increased quality grain yield of wheat.     

Exogenous double-stranded RNA inhibits the infection physiology of rust fungi to reduce symptoms in planta

Rust fungi (Pucciniales) are a diverse group of plant pathogens in natural and agricultural systems. They pose ongoing threats to the diversity of native flora and cause annual crop yield losses. Agricultural rusts are predominantly managed with fungicides and breeding for resistance, but new control strategies are needed on non-agricultural plants and in fragile ecosystems. RNA interference (RNAi) induced by exogenous double-stranded RNA (dsRNA) has promise as a sustainable approach for managing plant-pathogenic fungi, including rust fungi. We investigated the mechanisms and impact of exogenous dsRNA on rust fungi through in vitro and whole-plant assays using two species as models, Austropuccinia psidii (the cause of myrtle rust) and Coleosporium plumeriae (the cause of frangipani rust). In vitro, dsRNA either associates externally or is internalized by urediniospores during the early stages of germination. The impact of dsRNA on rust infection architecture was examined on artificial leaf surfaces. dsRNA targeting predicted essential genes significantly reduced germination and inhibited development of infection structures, namely appressoria and penetration pegs. Exogenous dsRNA sprayed onto 1-year-old trees significantly reduced myrtle rust symptoms. Furthermore, we used comparative genomics to assess the wide-scale amenability of dsRNA to control rust fungi. We sequenced genomes of six species of rust fungi, including three new families (Araucariomyceaceae, Phragmidiaceae, and Skierkaceae) and identified key genes of the RNAi pathway across 15 species in eight families of Pucciniales. Together, these findings indicate that dsRNA targeting essential genes has potential for broad-use management of rust fungi across natural and agricultural systems.     

Effect of plant stage, <Emphasis Type="Italic">Colletotrichum truncatum</Emphasis> dose,and use of herbicide on control of <Emphasis Type="Italic">Matricaria perforata</Emphasis>

Colletotrichum truncatum (Ct) was examined in a tank mix with the herbicide 2,4-D, clopyralid plus MCPA (Caurtail M^®), or metribuzin (Sencor^®) for control of scentless chamomile at 8- (younger) and 11-leaf stages (older) under controlled conditions. In initial trials, Ct at 7 × 10⁶ spores/ml (200 l/ha) reduced the fresh weight of scentless chamomile only slightly. However, its combinations with herbicides improved the efficacy variably depending on the herbicide used and stage of the weeds. Ct plus 2,4-D reduced the fresh weight by about 50% at both leaf stages of scentless chamomile when compared to untreated controls but no plants were killed. The fungus plus Curtail M consistently killed younger but not older plants, and the efficacy was substantially greater than that of the herbicide alone. The herbicide Sencor was highly effective on younger plants, and adding Ct did not achieve additional benefits. On older plants, however, Ct plus Sencor was substantially more effective than the herbicide alone, causing 76% fresh-weight reduction when compared to controls and killing 9 out of 16 older plants in four trials. Sencor applied alone reduced the fresh weight of older plants by 65%, but no plants were killed. Tested at doses ranging from 2 × 10⁶ to 20 × 10⁶ spores/ml, Ct plus Curtail M was most effective at the highest fungal inoculum dose, consistently killing younger but not older plants. In comparison, Ct at a medium dose (7 × 10⁶ spores/ml) plus Sencor killed the majority of older chamomile plants (7 out of 12), whereas the herbicide alone did not cause plant mortality. Further increasing fungal inoculum dose from this medium level did not enhance the weed control by Ct plus Sencor.     

Plant population and weeds influence stalk insects, soil moisture, and yield in rainfed sunflowers

Insect infestation, soil moisture, and yield were examined in populations of≈33 140 plants/ha （low） and ≈ 40 340 plants/ha （high） of an oilseed sunflower, Helianthus annuus L, cv. ＇ Triumph 660CL＇ with two levels of weediness. Less weedy plots resulted from the application of herbicide combination of S-metolachlor and sulfentrazone, whereas more weedy plots resulted from application of sulfentrazone alone. Among the 12 weed species recorded, neither plant numbers nor biomass differed between crop plant densities. Larvae of the stalk-boring insects Cylindrocopturus adspersus （Coleoptera： Curculionidae） and Mordellistena sp. （Coleoptera： Mordellidae） were less abundant in high density sunflowers, ostensibly due to reduced plant size. However, the same effect was not observed for Dectes texanus （Coleoptera： Cerambycidae） or Pelochrista womanana （Lepidoptera： Tortricidae）, two other stalk-boring insects. Soil moisture was highest in low density and lowest in the high density sunflowers that were less weedy. Stalk circumference, head diameter, and seed weight were reduced for sunflower plants with short interplant distances （mean = 20 cm apart） compared to plants with long interplant distances （mean = 46 cm apart）. These three variables were greater in less weedy plots compared with more weedy plot〉 and positively correlated with interplant distance. Yields on a per-hectare basis paralleled those on a per-plant basis but were not different among treatments. The agronomic implications of planting density are discussed in the context of weed and insect management.     

Effects of Intercropping on Growth and Reproductive Capacity of Late-emerging Senecio vulgaris L., with Special Reference to Competition for Light

Due to increased emphasis on long-term management of weed populationsin cropping systems with a reduced reliance on herbicides, theproduction of seeds by weeds that emerge after the criticalperiod for weed control is increasingly important. It was hypothesizedthat increased soil cover and light interception by a crop canopywould shorten the critical period for weed control and reducegrowth and fecundity of late-emerging weeds. This hypothesiswas tested in a series of field and glasshouse experiments inwhich competition for light was manipulated. Senecio vulgaris,an important weed in vegetable production systems, was chosenas the target plant, and canopies of pure and mixed stands ofleek and celery were used to provide shade. The time courseof light interception differed among the crop canopies. Increasingcompetition for light caused morphological changes to S. vulgaris,including a vertical shift in leaf area distribution. Increasedshading reduced biomass, capitula:shoot ratio and seed productionof S. vulgaris. However, the viability of seeds produced bythe shaded weed plants was not affected. Results indicate thatintercropping can increase light interception in a weakly competitivecrop such as leek and can contribute to weed suppression ina long-term strategy for weed management. Copyright 2001 Annalsof Botany Company Competition for light, late-emerging weeds, critical period, Apium graveolens L., celery, Allium porrum L., leek, Senecio vulgaris L., common groundsel, seed production, weed management, intercropping     

Interaction between crown rust (Puccinia coronata) and two viruses of ryegrass

In Lemtal Italian and S.24 perennial ryegrass plants, two isolates of ryegrass mosaic virus (RMV) suppressed the amount of crown rust emerging on leaves inoculated with Puccinia coronata uredospores by up to 75% compared with the amount on virus-free plants. Severity of rust infection on barley yellow dwarf virus (BYDV) infected plants generally did not differ significantly from that on virus-free plants. When both RMV and BYDV were present, rust was restricted in Lemtal plants to a level intermediate between those occurring on plants infected by either virus alone, and in S.24 plants to a level below that obtained with either virus alone. The mean water soluble carbohydrate (WSC) content of Lemtal plants was reduced more than 20% by RMV, but was not significantly altered by BYDV. In S.24 plants the WSC content was increased by 10% by RMV and by 60% by BYDV. Rust reduced the WSC content of healthy and virus-infected plants, the reduction being positively correlated with the level of rust on the sampled leaves. In plants of Lemtal, but not of S.24, the degree of rust infection was positively correlated with the WSC content of leaves from rust-free control plants.     

Induction of systemic protection against rust infection in broad bean by saccharin: effects on plant growth and development

Here, we examine the effect of saccharin on the induction of systemic resistance in broad bean (Vicia faba) to the rust fungus Uromyces viciae-fabae. Saccharin was applied to beans at the three-leaf stage, either as a soil drench or by painting the solution on to first leaves. Plants were then challenge inoculated with the rust 1, 6, 10 and 14 d following saccharin treatment, after which they were harvested, assessed for the intensity of rust infection and plant growth measurements conducted. Foliar application of saccharin did not induce systemic protection to rust infection until 14 d after application and was less effective than saccharin applied as a soil drench. When saccharin was applied as a drench, systemic protection was not observed until 6 d after application, but was still apparent in plants 14 d after application of the drench. Irrespective of the method of application, saccharin had no significant effect on fresh and dry weights or leaf area of the plants. Saccharin applied as a drench did, however, reduce the number of leaflets produced.     

The biology of Peronospora viciae on pea: the development of local and systemic infections and their effect on vining yield

The development of infection by Peronospora viciae (Berk.) Casp. was studied in field and plot experiments during 1967, 1968 and 1969. Numbers of primary systemically infected plants appeared shortly after emergence and reached a maximum by the end of May. Such plants acted as primary infectors and did not survive more than 3 weeks. Maximal sporangial release occurred between 06.00 and 08.00 h BST with a peak at 07.00 h, after 1–2 h insolation. Inoculation of leaves at 10.30 h resulted in greater infection than inoculation at 20.00 h. The result suggested a possible maturation requirement of sporangia following release from the sporangiophore. Initially the spread of foliar (local lesion) infection in the crop was sporadic and localized in cvs. Jade, Gregory, Dark Skinned Perfection (D.S.P.) and Frühe Kleine Pfälzerin (F.K.P.) during the 3 years. The number of plants of all cvs. with foliar infection reached 100% by mid-June. Lesions rarely exceeded 15% of the leaf and stipular area of the mature plant. No effect of foliar lesions, at the highest level of infection, in haulm or seed fresh weight could be detected in cvs. F.K.P., Jade and D.S.P. Secondary systemic infection occurred sporadically in field crops usually with dense haulm growth. Plants of Jade with systemic infection were rated according to the number of nodes affected. There was a small but significant increase in haulm and seed fresh weight in plants with up to two nodes affected. But plants with three to five nodes showed a progressive reduction in the yield of green seed (resulting from a reduction in the number of podding nodes) and in pod size and haulm fresh weight. Total losses in Jade during 1968 and 1969 with all grades of infection did not exceed 4 % of the green seed crop. Systemic infection in some plants resulted in the death of the apex and the production of healthy, normally podding tillers. Such plants had no commercial value due to delayed maturation. Pod infection in Jade occurred largely independently of systemic symptoms: the overall level during 1969 did not exceed 16 % and only 2 % were severely infected. Total losses (from both systemic and pod infection) in Jade during 1968 and 1969 represented only a negligible part of the crop.     

The biology and host specificity of the onion weed rust, Puccinia barbeyi, a potentially useful agent for biological control in Australia

Onion weed, Asphodelus fistulosus L., (Liliaceae) a weed of Mediterranean and Middle Eastern origin is widespread in southern Australia where it invades pastures making them unsuitable for grazing. A program of research is underway to discover natural enemies of this plant and to study their possible role in the biological control of onion weed. A rust fungus Puccinia barbeyi (Roum.) Magnus has been found to severely attack A. fistulosus . Observations on the biology of the rust confirmed that it is monoecious and microcyclic and multiplied essentially by aecial and telial stages, although occasionally urediniospores also appeared among teliospores. Several members of the Liliaceae exposed to the aeciospores of the rust remained unattacked indicating that it is most probably specific to Asphodelus spp. and thus its potential for the biological control of A. fistulosus in Australia should be studied further.