Dutch elm disease in New Zealand: impacts from eradication and management programmes

Ophiostoma novo‐ulmi, the cause of the current pandemic of Dutch elm disease, is considered one of the 20 worst pests to have been introduced to New Zealand. An attempt to eradicate the pathogen came close to success, but was eventually stopped and replaced by management programmes. These programmes are slowing the spread of the pathogen. If left uncontrolled it is likely that New Zealand will lose 90% of its elm trees. Although elms are exotic trees in New Zealand, the loss of such a large number of popular amenity trees is likely to have numerous social, cultural, environmental and economic effects. It is estimated that this disease could cost public and councils over NZD $350 million in tree removal and replacement costs. Control of O. novo‐ulmi is limited, but biological control using d‐factor viruses has been proposed. Although O. novo‐ulmi was not eradicated from New Zealand, the eradication programme had many successes; in particular, the effective interaction of good management and science in the early years of the campaign. Also highlighted in this review are several tactics and steps that could be applied to eradication attempts for other species.     

Resistance levels of Spanish conifers against Fusarium circinatum and Diplodia pinea

Pitch canker, caused by Fusarium circinatum, and Diplodia shoot blight, caused by Diplodia pinea, are both damaging to pines (Pinus spp.) grown in plantations throughout the world, including Spain. To assess the potential for interspecific differences in susceptibility to contribute to the management of pitch canker and Diplodia shoot blight in the Atlantic region of Spain, the present study was undertaken to characterize the susceptibility of six pine species (P. sylvestris, P. nigra, P. pinaster, P. radiata, P. halepensis and P. pinea) and Douglas‐fir (Pseudotsuga menziesii) to F. circinatum and D. pinea. Based on inoculations of 2‐year‐old trees, Ps. menziesii, P. pinea and P. nigra were the most resistant to F. circinatum, with lesion lengths ranging from 3.7 to 21.5 mm, 2.2 to 12.6 mm and 2.8 to 30.9 mm, respectively. At the other extreme, Pinus radiata was the most susceptible, sustaining lesions that ranged from 8.5 to 74.8 mm in length. Pinus sylvestris, P. pinaster and P. halepensis showed an intermediate response to F. circinatum. Broadly similar results were observed in inoculations with D. pinea, with Ps. menziesii being relatively resistant and P. radiata being highly susceptible. Consistent with these results, field surveys revealed no pitch canker in stands of Ps. menziesii and low severity of Diplodia shoot blight, whereas P. radiata was severely affected by both diseases. Our findings suggest that selection of appropriate species can greatly reduce the risk of damage from two important canker diseases affecting pine plantations in the Atlantic region of Spain. Furthermore, intraspecific variation in susceptibility implies that selection may allow for the enhancement of resistance in otherwise susceptible species.     

Endophyte-mediated resistance against white pine blister rust in Pinus monticola

Induced resistance responses, including fungal endophyte-mediated resistance, have been well studied in both agricultural crops and grass systems. Yet, the effect of these processes and symbionts in forest trees is poorly known. Fungal endophytes have been found in all conifer forest systems examined to date and have been hypothesised to be involved in resistance-mediated responses. However, in the absence of functional studies the influence of these endophytes on the extended phenotype of the host plant is unclear. In this study we demonstrate that fungal endophytes from Pinus monticola were effective at increasing survival in host plants against the exotic pathogen Cronartium ribicola, which is responsible for the devastating disease called white pine blister rust. Seedlings previously inoculated with fungal endophytes lived longer than endophyte-free seedlings and also showed some reduction in white pine blister rust disease severity. This endophyte-mediated resistance was found to be effective over time, indicating persistence, and is hypothesised to be a form of induced resistance. Overall, this suggests fungal endophytes may play a determinative role in the structure of biological communities and could provide a useful alternative or ancillary management tool for combating pests and diseases.     

The hemibiotrophic lifestyle of the fungal pine pathogen Dothistroma septosporum

Dothistroma needle blight is a forest disease of increasing international importance due to its ability to kill as well as to retard the growth of pines. It is caused by fungi in the genus Dothistroma that produce dothistromin, a non‐host selective toxin and virulence factor that is involved in necrosis of pine tissue. Recent studies of the genome of one of the main pathogenic species, Dothistroma septosporum, showed that it contains many similarities to that of the biotroph Cladosporium fulvum, including the presence of candidate biotrophic effector genes, which supported the hypothesis that D. septosporum has a hemi‐biotrophic lifestyle. Using Pinus radiata as a host, we used a combination of microscopy, histological and molecular tools to further test this hypothesis and to determine the stage of the disease cycle in which dothistromin toxin is produced. The results showed a biotrophic‐type phase in which the fungus grew over the needle surface, penetrated through stomatal pores and colonized epistomatal chambers. The subsequent necrotrophic phase was characterized by colonization of the mesophyll and production of dothistromin, with a >100‐fold increase in dothistromin levels from early necrotic lesion to sporulating lesion stages. This is consistent with the role of dothistromin as a virulence factor that is involved in lesion expansion.     

Dothistromin toxin is a virulence factor in dothistroma needle blight of pines

Dothistromin is a broad‐spectrum mycotoxin produced by the Dothideomycete pine needle pathogen Dothistroma septosporum. It accumulates in lesions, causing characteristic red bands on needles infected with this fungus. Dothistromin is similar in structure to the aflatoxin precursor versicolorin B and the biosynthetic pathways of these toxins share many common gene products. Although dothistromin is not essential for pathogenicity in dothistroma needle blight, its presence in infected needles suggests it might have a role in the disease process. The hypothesis that dothistromin is a virulence factor was tested by studying Pinus radiata infected with dothistromin‐deficient mutants of D. septosporum. The mutants were able to infect pine needles and complete their life cycle as previously shown, and were unaffected in spore germination, epiphytic growth or needle penetration. However, colonization of the mesophyll by the mutants was restricted compared to the wild type. Correspondingly, lesions produced by the mutants were smaller and produced significantly fewer spores than lesions produced by wildtype strains. Interestingly, ‘green islands’, in which chlorophyll was maintained at a higher level than in adjacent chlorotic and necrotic regions, surrounded early‐appearing lesions caused by both wildtype and mutant strains. At a later stage of disease development green islands were still present in the mutant but appeared to be masked by the extended dothistromin‐containing lesions in the wild type. Overall the results support a role for dothistromin in virulence in dothistroma needle blight.     

Susceptibility of native New Zealand Myrtaceae to the South African strain of Austropuccinia psidii: A biosecurity threat

Austropuccinia psidii, cause of myrtle rust, has spread globally where Myrtaceae occur. Multiple strains of A. psidii have been identified, including a unique strain found only in South Africa. The South African strain is a biosecurity concern for species of Myrtaceae worldwide. This is because preliminary testing of South African Myrtaceae suggests it could have a wide host range, and thus has the potential to be invasive. In this study, we assessed the ability of the South African strain to infect other species of Myrtaceae by testing the susceptibility of New Zealand provenance Myrtaceae. Seedlings of four native New Zealand Myrtaceae species (Metrosideros excelsa, Leptospermum scoparium, Kunzea robusta, and Kunzea linearis) were artificially inoculated in South Africa with a single-uredinium isolate of the South African strain. Fourteen days after inoculation, uredinia, and in many cases telia, had developed on the young leaves and stems of all four host species, which led to shoot tip dieback in the more severe cases. When comparisons were made between the species, K. robusta was the least susceptible to the South African strain of A. psidii, while L. scoparium and M. excelsa were the most susceptible. While only a limited number of seed families were tested, only a small proportion of the seedlings showed resistance to infection by the South African strain. This preliminary testing highlights the potential invasive risk the South African strain poses to global Myrtaceae communities, including New Zealand Myrtaceae.