Puccinia psidii, the cause of a disease today commonly referred to as Myrtle rust, is considered a high priority quarantine threat globally. It has a wide host range in the Myrtaceae and it is feared that it may result in significant damage to native ecosystems where these plants occur. The fungus is also of considerable concern to plantation forestry industries that propagate Australian Eucalyptus species. In May 2013, symptoms of a rust disease resembling those of P. psidii were observed on an ornamental Myrtaceous shrub in a garden in South Africa. The fungus was identified based on DNA sequence data of the ITS and 5.8S nrRNA gene regions and here we report, for the first time, the presence of P. psidii in Africa.
Fusarium circinatum is an important fungal pathogen of Pinus species. In South Africa, it is the most significant pathogen of Pinus patula seedlings in forestry nurseries where it presents a substantial constraint to productivity and can continue to cause mortality in-field for up to two years after establishment. This study describes the results from two trials where P. patula seedlings were inoculated with F. circinatum to determine the impact of the pathogen on nursery and field performance. Seedlings were also subjected to water stress treatments to ascertain whether this would trigger the onset of disease symptoms. Inoculum load and timing of inoculation had significant effects on seedling survival in both the nursery and field. High inoculum concentrations caused greater levels of mortality and, where seedlings were inoculated at a young age, they showed higher levels of susceptibility to F. circinatum. Temporary water-stress in the nursery produced smaller plants and improved in-field survival, but this treatment did not trigger higher mortality in inoculated treatments. On the other hand, transplant stress was a major contributor to the higher levels of mortality observed in inoculated treatments. Overall, these studies confirmed that infection in the nursery leads to the disease problems observed during early plant establishment in the field.
Fusarium circinatum is a pathogen causing serious post-planting mortality of Pinus patula seedlings in southern Africa. Containerized planting stock that is asymptomatic but associated with F. circinatum in the nursery is thought to be the cause of this problem. The aim of this study was to determine if re-use of seedling containers could be a source of inoculum resulting in asymptomatic planting stock and increased post-planting mortality of Pinus patula. Two experiments were conducted in successive years comparing nursery cull of symptomatic seedlings, seedling growth, association of F. circinatum with asymptomatic seedlings and post-planting mortality for crops raised in re-used containers, with and without sanitation, and factory new containers. Each experiment consisted of a nursery production trial followed by out-planting into pots to assess post-planting mortality. Our results show that re-use of containers without sanitation increases the cull of symptomatic seedlings, incidence of F. circinatum associated with asymptomatic seedlings and post-planting mortality compared to the re-use of containers after steam sanitation or factory new containers. Growth of asymptomatic seedlings was unaffected by container treatment or association with F. circinatum and in the absence of wilt symptoms the root system did not exhibit typical discolouration. Watering frequency did not influence post-planting mortality in pots. The comparison of two open-pollinated seed mixes of P. patula that, based on seedling stem inoculation screening, represented susceptible and tolerant material did not show differences in nursery cull or post-planting mortality. This work demonstrated that natural contamination of re-used containers can be a primary source of inoculum producing asymptomatic seedlings associated with F. circinatum that will succumb to the pathogen after field planting. The process of seedling infection, apparent latent infection in the seedling and expression of disease after planting needs greater understanding to improve nursery hygiene measures to control this disease.
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