Fs Hay

Diseases of Pyrethrum in Tasmania: Challenges and Prospects for Management

Pyrethrum (Tanacetum cinerariifolium (Trevir.) Sch. Bip.) is a perennial plant and member of the Asteraceae that is endemic to the Dalmatian region of the former Yugoslavia (36). Pyrethrum is cultivated commercially solely for the production of six closely related esters called pyrethrins. The plant is tufted, slender, and herbaceous, growing to a height of approximately one meter (18). Leaves are alternate and pinnately lobed/narrowly lanceolate to oblong lanceolate. The daisy-like flowers are produced at the termini of stems and consist of a cluster of 40 to 100 bisexual, yellow disk florets encircled by a ring of 18 to 22 pistillate white ray florets atop a moderately convex to subglobose receptacle (Fig. 1; 100). Disk and ray florets both possess 3 to 10 ribbed achenes located between the floret and receptacle. Involucres generally range between 12 and 18 mm in diameter (17,18). Approximately 94% of the pyrethrins are produced within secretory ducts and oil glands of the achenes of the mature pyrethrum flower, with a minor percentage of oil glands and secretory ducts also found in leaves, stems, and roots (99). Pyrethrins can be separated into two groups of three ester compounds: pyrethrins I and II. The pyrethrin I fraction contains chrysanthemic acid products, including pyrethrin I, cinerin I, and jasmolin I. The pyrethrin II fraction is derived from pyrethric acid and made up of pyrethrin II, cinerin II, and jasmolin II (19,27). Compounds within both fractions contain insecticidal properties used in household and commercial pest control products. These compounds are referred to as “knockdown” and kill agents for many arthropods, yet are of low toxicity to mammals. Pyrethrins also have the advantage over other synthetic insecticides of being rapidly broken down upon exposure to light and air, are metabolized quickly, and can be used in the production of organic farm products. Thus, natural pyrethrins are generally considered to be nonpolluting (19,27). The major areas of pyrethrum production worldwide are located in East Africa (Kenya, Rwanda, and Tanzania), Tasmania (Australia), China, and Papua New Guinea (85,94). Production of pyrethrum in Kenya began in 1928, and despite some fluctuations in annual supply, Kenya is still one of the major suppliers to the world’s market. Pyrethrum cultivation in Kenya is centered in four production areas: the northern and southern Rift Valleys, Mount Kenya, and near Lake Victoria (94). Tasmania is the other major world producer and grows approximately 2,000 hectares. In Tasmania, pyrethrum is grown predominantly along the northwest coast of the island, between Deloraine (41° 31′ S; 146° 39′ E) and Table Cape (40° 56′ S; 145° 43′ E). The cultivation of pyrethrum differs markedly between Tasmania and the other production areas of the world. For example, in 2001 approximately 200,000 growers were involved in pyrethrum production

Spatiotemporal Description of Epidemics Caused by Phoma ligulicola in Tasmanian Pyrethrum Fields

ABSTRACT Spatial and temporal patterns of foliar disease caused by Phoma ligulicola were quantified in naturally occurring epidemics in Tasmanian pyrethrum fields. Disease assessments (defoliation incidence, defoliation severity, incidence of stems with ray blight, and incidence of flowers with ray blight) were performed four times each year in 2002 and 2003. Spatial analyses based on distribution fitting, runs analysis, and spatial analysis by distance indices (SADIE) demonstrated aggregation in fields approaching their first harvest for all assessment times between September and December. In second-year harvest fields, however, the incidence of stems with ray blight was random for the first and last samplings, but aggregated between these times. Spatiotemporal analyses were conducted between the same disease intensity measures at subsequent assessment times with the association function of SADIE. In first-year harvest fields, the presence of steep spatial gradients was suggested, most likely from dispersal of conidia from foci within the field. The importance of exogenous inoculum sources, such as wind-dispersed ascospores, was suggested by the absence of significant association between defoliation intensity (incidence and severity) and incidence of stems with ray blight in second-year harvest fields. The logistic model provided the best temporal fit to the increase in defoliation severity in each of six first-year harvest fields in 2003. The logistic model also provided the best fit for the incidence of stems with ray blight and the incidence of flowers with ray blight in four of six and three of six fields, respectively, whereas the Gompertz model provided the best fit in the remaining fields. Fungicides applied prior to mid-October (early spring) significantly reduced the area under disease progress curve (P < 0.001) for defoliation severity, the incidence of stems with ray blight, and the incidence of flowers with ray blight for epidemics at all field locations. This study provides information concerning the epidemiology of foliar disease and ray blight epidemics in pyrethrum and offers insight on how to best manage these diseases.

Development of a Fungicide-Based Management Strategy for Foliar Disease Caused by Phoma ligulicola in Tasmanian Pyrethrum Fields

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.

Seasonal fluctuations in fungi associated with pyrethrum foliage in Tasmania

The temporal fluctuations in disease severity (frequency of plants in disease categories) and the frequency of fungal isolations from lesions on foliage of pyrethrum (Tanacetum cineariifolium) were recorded in fields approaching their first harvest throughout 2000 and 2001 in northern Tasmania. In both years, the number of plants with areas of necrotic foliage in the lesser disease categories (< 0.50 of foliage affected) was significantly higher than expected by random chance until August. However, in both years from September onwards this trend was reversed, with the number of plants with areas of necrotic foliage in the higher disease categories (representing > 0.50) being significantly higher than expected by random chance. This corresponded with an increase in the number of consecutive days with rain of at least 0.1 mm and an increase in the frequency of isolation of Phoma ligulicola (a known pathogen) from all lesion types. P. ligulicola was also the most commonly isolated fungus from lesions on stems and buds in 2000 and from necroses of all tissues in 2001. In 2001, necrotic lesions on leaves were differentiated into necrotic spots or marginal necrotic lesions (extending from the margin of the leaf). Alternaria alternata was mostly commonly associated with marginal necroses. A. tenuissima, Stemphylium botryosum and P. ligulicola were the most common fungi associated with necrotic spots on leaves.

Pathogenicity of fungi commonly isolated from foliar disease in Tasmanian pyrethrum crops

The pathogenicity of Alternaria alternata, A. tenuissima, Stemphylium botryosum, Cladosporium cladosporioides and Ulocladium atrum commonly associated with diseased foliage of pyrethrum (Tanacetum cineariifolium) was characterised by inoculating glasshouse-grown pyrethrum plants in three replicated experiments. In the first experiment only one cultivar was used, whilst in the remaining two experiments the susceptibility of cultivars was evaluated. Inoculation with A. alternata, C. cladosporioides and U. atrum failed to induce disease in any cultivar. Inoculation with A. tenuissima resulted in the development of necrotic spots which coalesced to cover large areas of the leaves in all cultivars, whereas foliage of those inoculated with S. botryosum developed necrotic halos surrounded by a pink/brown margin. No significant differences were detected in disease severity following inoculation with A. tenuissima or S. botryosum between the four cultivars commonly grown in Tasmania. However, some significant differences were noted between cultivars in the number of lesions per leaf following inoculation with S. botryosum.

Seedborne Infection of Pyrethrum by Phoma ligulicola

The incidence of Phoma ligulicola in Tasmanian pyrethrum seed and methods of managing seedborne mycoflora were determined. Fourteen different fungi were regularly isolated from seed, including Alternaria tenuissima, Stemphylium botryosum, and P. ligulicola, which have been documented as pathogens of pyrethrum. Comparisons between the incidence of these fungi from seed surface-sterilized with sodium hypochlorite and nontreated seed indicated they occurred both within and on the outside of the seed. A polymerase chain reaction (PCR) test for the detection of P. ligulicola was also developed, with a detection limit of 800 fg of fungal DNA. The assay detected infested seed lots down to an incidence of 0.5%. Reliable amplification of the target DNA was achieved with the addition of bovine serum albumin to reduce the influence of inhibitors from pyrethrum seed. Agar plate tests and PCR demonstrated variability with pyrethrum cultivars in the presence and viability of P. ligulicola in seed. Effective management strategies for the reduction of seedborne P. ligulicola included the regular use of fungicides for reducing foliar disease intensity in the seed fields prior to harvest. Seed treatments with fludioxonil and thiabendazole/thiram also significantly reduced the incidence of seedborne P. ligulicola and increased seed germination and seedling survival.

Phylogenetic analysis of the downy mildew pathogen of oilseed poppy in Tasmania, and its detection by PCR

Downy mildew of oilseed poppy (Papaver somniferum) has become a serious disease issue for the Tasmanian poppy industry since its first record in 1996. Previous reports have reported the pathogen as Peronospora arborescens, which is differentiated from the related species P. cristata, also known to infect Papaver spp., by conidium dimensions alone. This study investigated the taxonomic status of the downy mildew pathogen, using both morphological characters and molecular analysis of the internal transcribed spacer (ITS) region of the ribosomal DNA (rDNA). The inherent variability of conidium dimensions made differentiation of species difficult. Sequence homology and phylogenetic analyses of the ITS region showed the pathogen to be more closely related to P. cristata than P. arborescens. It is therefore proposed that downy mildew of oilseed poppy in Tasmania be reattributed to the pathogen P. cristata. In addition to this work, PCR primers have been developed for the specific detection of the downy mildew pathogen in Tasmania.

Genetic relationships among isolates of Phoma ligulicola from pyrethrum and chrysanthemum based on ITS sequences and its detection by PCR

Variation within the internal transcribed spacer (ITS 1, 5.8S gene and ITS2) region of the rDNA (ITS) was used to characterise the phylogenetic relationships among Phoma ligulicola isolates infecting pyrethrum crops in Tasmania, P. ligulicola isolates from the USA, Germany and mainland Australia, and other closely related fungal species. This study reports the first characterisation of the ITS region of P. ligulicola. Sequence homology within P. ligulicola isolates varied between 99.3 and 100%. For 9 of the 11 isolates from Tasmania, Australia, the nucleotide sequences in this region were identical, whereas the sequences for the remaining two isolates differed only by two nucleotides in the ITS 1 region. Isolates from Australia and the USA failed to metabolise NaOH on malt-extract agar and were characterised as P. ligulicola var. inoxydablis. The two isolates from ray blight disease of chrysanthemum in Germany (DSMZ 63133 and DSMZ 62547) were classified as P. ligulicola var. ligulicola. Phylogenetic analyses suggested that the ITS sequences of P. ligulicola isolates were more similar to other Phoma species than selected representatives of the Mycosphaerella genus. Didymella bryoniae had the greatest interspecific homology with P. ligulicola of the fungi used in this study. This information was used to design specific primers within the ITS regions for the detection of P. ligulicola.

Influence of Phoma ligulicola on yield, and site factors on disease development, in Tasmanian pyrethrum crops

Phoma ligulicola Boerema was isolated from diseased buds, and necrotic stem and leaf lesions in 24 pyrethrum fields approaching their first harvest in northern Tasmania. Despite the ubiquitous distribution of this pathogen, marked differences in disease severity were evident in several fields. A higher incidence of disease was noted in fields with a southerly exposure and low areas within fields than on sites with a northerly exposure and hill crests. In 52% of the fields surveyed, P. ligulicola was associated with reduced yields of pyrethrin (kg ha−1 active ingredient) of up to 100%.

Spatiotemporal Analysis of Epiphytotics of Downy Mildew of Oilseed Poppy in Tasmania, Australia

Downy mildew, caused by Peronospora arborescens, has become the major disease affecting oilseed poppy (Papaver somniferum) since its first record in Tasmania in 1996. Two field trials conducted in 2000 and 2001 studied the progression and spatial distribution of downy mildew epiphytotics. The logistic and exponential models best described the progression of disease incidence and severity, respectively. Incidence and severity increased rapidly following canopy closure. In 2001, incidence increased from 0.16%, prior to canopy closure, to 100% at late flowering (40 days). Spatial analyses of epiphytotics were conducted by fitting the beta-binomial and binomial distributions, median runs analysis, and the spatial analysis by distance indices. All analyses demonstrated that the distribution of incidence and severity was strongly spatially aggregated from canopy closure until at least late flowering. These results suggest that secondary spread from a few primary infections is the major factor in epiphytotics.