Seedborne Infection of Pyrethrum by Phoma ligulicola

Abstract

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.

Diseases of Pyrethrum in Tasmania: Challenges and Prospects for Management

“…In contrast, seed is considered the most likely source of primary inoculum for P. ligulicola on pyrethrum. Pethybridge et al (69) found all seed lots of pyrethrum tested in Tasmania were infested with P. ligulicola, with up to 28% incidence of infested seeds per lot. Since this finding, the application of foliar fungicides to pyrethrum seed crops, and as a seed treatment prior to planting, has significantly reduced the incidence of viable P. ligulicola in seed.…”

“…Alternatively, the fungus may be homothallic but environmental conditions in Tasmania may not be conducive for teleomorph production. In the absence of evidence of the teleomorph, epidemics at remote locations in Tasmania have been attributed to inoculum introduced on infested seed (69).…”

Site-Specific Risk Factors for Ray Blight in Tasmanian Pyrethrum Fields

“…Earlier studies had demonstrated a significant linear relationship between defoliation severity within spring crops and the incidence of P. ligulicola var. inoxydablis in the harvested seed (39). The ability to assess the role of this source of primary inoculum in subsequent epidemics was not possible in this study since all seedlots had at least 1% incidence of P. ligulicola var.…”

“…Testing of seed to quantify the incidence of seed infection by P. ligulicola var. inoxydablis was conducted using methodology described by Pethybridge et al (39). Using global positioning systems and geographic information systems software, the centroids of each pyrethrum field were used to obtain interpolated site-specific daily weather variables (temperature and rainfall) from the Queensland Department of Natural Resources and Mines Data Drill algorithm (17).…”

Minimizing Crop Damage Through Understanding Relationships Between Pyrethrum Phenology and Ray Blight Disease Severity

“…A spring fungicide program was introduced in 2002 in response to severe losses incurred from annual epidemics (20), In replicated trials conducted in 2002 and 2003, fungicide applications resulted in a doubling of the number of flowers produced, leading to significant increases in pyrethrin yield (24). Currently, the program is dependent upon the suecinate dehydrogenase inhibitor boscalid, which provides effective control of ray blight (18) as well as Selerotinia crown rot caused by Sclerotinia minor and S. sclerotiorum (18,22). A range of fungicide chemistries have been used within the program to date, of which some have developed resistance and, hence, have been discarded frotn the program (9).…”