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Production of the phytotoxin thaxtomin A by pathogenic Streptomyces spp. is essential for induction of common scab disease in potato. Prior studies have shown that foliar application of sublethal concentrations of 2,4-dichlorophenoxyacetic acid (2,4-D) and other auxin or auxin-like compounds significantly reduced severity and occurrence of common scab in subsequently produced tubers. However, the means of disease suppression by these compounds was not known. We confirm the disease suppressive activity of 2,4-D. Detailed tuber physiological examination showed that lenticel numbers, lenticel external dimensions, and periderm thickness and structure, physiological features believed to be critical to Streptomyces scabiei infection, were not substantially changed by 2,4-D treatments, negating a possible mechanism for disease suppression through alteration of these structures. In contrast, our studies show accumulation of 2,4-D in tubers of treated plants occurs and is associated with an enhanced tolerance to thaxtomin A. Applying 2,4-D to cultures of S. scabiei did not significantly alter in vitro growth of the pathogen. Thaxtomin A production by the pathogen was inhibited by 2,4-D, but only at the highest rate tested (1.0 mM), which is at least 200-fold more than is found in 2,4-D treated tubers. These data suggest 2,4-D has no direct effect on the pathogen or its virulence. Confirmatory evidence from studies with Arabidopsis thaliana seedlings demonstrated that the auxins 2,4-D and IAA ameliorate thaxtomin A toxicity. The evidence presented whereby auxin treatment inhibits toxicity of thaxtomin A secreted by the pathogen suggests a novel indirect means of disease suppression.

Thaxtomin A, a key phytotoxin produced by plant pathogenic Streptomyces sp., is implicit in common scab disease expression in potato. Primary targets and modes of action of thaxtomin A toxicity in plant cells are not well understood. In this work, early signalling events associated with thaxtomin A toxicity were studied using the ion-selective microelectrode ion flux estimation (MIFE) technique. Thaxtomin A-induced changes in net ion fluxes were measured across the plasma membrane (PM) of root and pollen tube tissue in Arabidopsis thaliana and tomato. Within a minute after toxin application, a rapid and short-lived Ca2+ influx was observed. Well ahead of the marked inhibition of root growth, a significant shift towards net H+ efflux across the PM occurred in all tissues. Similar to root tissues, thaxtomin A significantly modified ion flux profiles from growing pollen tubes. Thaxtomin A was more effective in young, physiologically active tissues (root elongation zone or pollen tube apex), suggesting a higher density of thaxtomin A-binding sites in these regions. Overall, our data provide the first evidence that thaxtomin A triggers an early signalling cascade, which may be crucial in plant-pathogen interactions. It also suggests a possible interaction between thaxtomin A and PM auxin receptors, as revealed from experiments on the auxin-sensitive ucu2-2/gi2 A. thaliana mutant.

The importance of both seed and soil‐borne inoculum in the epidemiology of common scab disease under Australian conditions was clearly demonstrated. In field trials the severity of disease in harvested potatoes was directly related to the severity of disease on the planted seed tubers. Chemical seed dressing treatments were assessed for common scab disease control under field conditions in four trials over 5 years. Where seed treatments were applied to both diseased and visibly clean seed pieces significantly more disease was found in tubers harvested from diseased seed than the corresponding clean seed treatment. In all but one trial, the treatments applied to diseased seed significantly reduced the incidence of common scab. Fluazinam, flusulfamide (at elevated rates), fenpiclonil, pentachloronitrobezene and mancozeb seed treatments were particularly effective as seed dressing treatments. Applications to visibly clean seed failed to significantly diminish disease levels below that found on untreated seed. Preliminary investigations of some chemical soil treatments gave disappointing levels of control.

Seed is the main source of infection of narrow-leafed lupin (Lupinus angustifolius) crops by cucumber mosaic virus (CMV). The ELISA procedure is currently used for large-scale, routine testing of lupin seed samples, but a more sensitive, reliable and labour-saving assay is needed which detects levels of seed infection as low as 0.1%. A Polymerase Chain Reaction (PCR) using ground dry seed samples was developed for this purpose. Primers based on concensus sequences of eight published CMV coat protein cDNAs (RNA3) of CMV subgroups 1 and 2 were used. The assay involved (1) a reverse transcription step for cDNA synthesis and (2) amplification of a specific fragment (482-501 bp depending on the strain) by PCR. Two methods of extracting virus from infected lupin material were used: (i) a rapid procedure which was effective for samples with higher levels of infection, e.g. infected leaves and 20.5% infected seed; (ii) a phenol-chloroform procedure, which led to greater sensitivity, enabling reliable detection of 0.1% seed infection. It detected CMV in 16 commercial seed samples (0.1-8% seed infection) belonging to seven cultivars from 12 different localities. Both methods were suitable for routine testing of the flour derived from grinding dry seed. On dissection of infected seeds, CMV was detected in the cotyledons and embryo and usually in or on the testa. The PCR assay detected virus from both CMV subgroups, but only subgroup 2 was found in lupin seed samples. The two CMV subgroups can be distinguished by digestion of amplified DNA with the restriction enzyme EcoRI; only CMV strains of subgroup 2 are digested to yield two fragments of size 330 and 170 bp.

The efficacy of a range of fungicides were assessed under in vitro conditions in four field trials over three seasons for the management of ray blight of pyrethrum, caused by the fungus Phoma ligulicola in Tasmania. Fungicides belonging to the demethylation inhibitor group, including prochloraz, tebuconazole, difenoconazole, and cyproconazole, produced the most significant reduction in mycelial growth in vitro compared with unamended controls. Significant reductions in germination of conidia resulted from low concentrations of azoxystrobin. In the field, azox-ystrobin (as Amistar) at a rate of 300 g of product/ha significantly decreased disease intensity and increased pyrethrin yield (kg/ha) in all seasons. Applications of either azoxystrobin or difenoconazole (as Score) at a rate of 250 ml of product/ha in early spring were the most effective, whereas no significant benefit was demonstrated from applications in autumn. Over all seasons, azoxystrobin applications resulted in double the weight of flowers produced compared with nontreated areas, whereas increases in pyrethrin content within the flowers were season and location specific. Application of azoxystrobin and chlorothalonil (as Bravo 720) at a rate of 1.4 liters of product/ha resulted in a 45 to 48% increase in flowers, reduced defoliation, and increased stem diameter. The weight of individual flowers and flower maturity was not significantly affected by fungicide application. These data provide the basis for cost-effective management of this disease.

Male-male competition in plants is thought to exert selection on flower morphology and on the temporal presentation of pollen. Theory suggests that a plant's pollen dosing strategy should evolve to match the abundance and pollen transfer efficiency of its pollinators. Simultaneous pollen presentation should be favored when pollinators are infrequent or efficient at delivering the pollen they remove, whereas gradual dosing should optimize delivery by frequent and wasteful pollinators. Among Penstemon and Keckiella species, anthers vary in ways that affect pollen release, and the morphology of dried anthers reliably indicates how they dispense pollen. In these genera, hummingbird pollination has evolved repeatedly from hymenopteran pollination. Pollen production does not change with evolutionary shifts between pollinators. We show that after we control for phylogeny, hymenopteran-adapted species present their pollen more gradually than hummingbird-adapted relatives. In a species pair that seemed to defy the pattern, the rhythm of anther maturation produced an equivalent dosing effect. These results accord with previous findings that hummingbirds can be more efficient than bees at delivering pollen.

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