Research into crown rot and wilt diseases of strawberries in Queensland

Plant losses due to fungal diseases in strawberry (Fragaria × ananassa Duch.) can potentially cause total loss of production. Three fungal pathogens, Fusarium oxysporum f. sp. fragariae, Colletotrichum gloeosporioides and Macrophomina phaseolina, cause similar crown rot and wilt symptoms in strawberries in Queensland. Since the withdrawal of methyl bromide in 2005, no effective chemical control of any of the three pathogens has been available. This study aims at identifying sources of plant genetic resistance that can be used in the breeding program to develop resistant cultivars for use as part of an integrated disease management plan in commercial strawberry production. Results from glasshouse pathogenicity and screening trials on the three pathogens showed that when breeding for resistance against a pathogen, resistance to other pathogens also needs to be considered, e.g., cultivar 'Festival' is resistant to F. oxysporum f. sp. fragariae, but is highly susceptible to C. gloeosporioides. The cultivars 'Earlisweet', 'Kabarla' and 'Phenomenal', two seedling clones and four DAFF breeding lines were resistant to C. gloeosporioides. Cultivar 'Suncoast Delight' showed the most promising level of resistance to M. phaseolina. These cultivars, breeding lines and seedling selections have been made available for incorporation into the crossing program to support the Queensland strawberry breeding program.

Mealybug wilt disease

Mealybug wilt disease (MWD) is a serious field disease of pineapples worldwide that was first described in Hawaii in 1910. MWD is thought to be caused by a complex involving viruses, mealybugs and ants. The viruses are transmitted by mealybugs, which in turn are tended by ants. Although a number of distinct viruses have been associated with the disease, the identity of the causal agent(s) has not been determined unequivocally. This chapter describes the disease symptopms, aetiology and management of MWD. In the last 20 years, significant advances have been achieved in identifying the causal viral agents, and gaining a better understanding of MWD. However, the interactions between the viruses, mealybugs and environmental factors are complicated, and the conditions required for the expression of MWD have only been partially elucidated at this time. The possible role of gene silencing, the identity of the additional ampelovirus(es) and badnavirus(es) that have been detected but not characterized, and the interaction between these disease-inducing factors are fertile areas for future research.

Comparison of growth traits between abundant and uncommon forms of a non-native vine, Dolichandra unguis-cati (Bignoniaceae) in Australia

Cat’s claw creeper vine, Dolichandra unguis-cati (L.) Lohmann (syn. Macfadyena unguis-cati (L.) Gentry) (Bignoniaceae), is a major environmental weed in Australia. Two distinct forms of this weed (‘long’ and ‘short’ pod), with differences in leaf morphology and fruit size, occur in Australia. The long pod form has only been reported in less than fifteen localities in the whole of south-east Queensland, while the short pod form is widely distributed in Queensland and New South Wales. This study sought to compare growth traits such as specific leaf area, relative growth rate, stem length, shoot/root ratio, tuber biomass and branching architecture between these forms. These traits were monitored under glasshouse conditions over a period of 18 months. Short pod exhibited higher values of relative growth rates, stem length, number of tubers and specific leaf area than long pod, but only after 10 months of plant growth. Prior to this, long and short pod did not differ significantly. Higher values for these traits have been described as characteristics of successful colonizers. Results from this study could partly explain why the short pod form is more widely distributed in Australia while long pod is confined to a few localities.

What influences fungal communities in cotton soils

Soilborne diseases such as Fusarium wilt, Black root rot and Verticillium wilt have significant impact on cotton production. Fungi are an important component of soil biota with capacity to affect pathogen inoculum levels and their disease causing potential. Very little is known about the soil fungal community structure and management effects in Australian cotton soils. We analysed surface soils from ongoing field experiments monitoring cotton performance and disease incidence in three cotton growing regions, collected prior to 2013 planting, for the genetic diversity and abundance as influenced by soil type, environment and management practices and link it with disease incidence and suppression. Results from the 28S LSU rRNA sequencing based analysis indicated a total of 370 fungal genera in all the cotton soils and the top 25 genera in abundance accounted for the major portion of total fungal community. There were significant differences in the composition and genetic diversity of soil fungi between the different field sites from the three cotton growing regions. Results for diversity indices showed significantly greater diversity in the long-term crop rotation experiment at Narrabri (F6E) and experiments at Cowan and Goondiwindi compared to the Biofumigation and D1 field experiments at ACRI, Narrabri. Diversity was lowest in the soils under brassica crop rotation in Biofumigation experiment. Overall, the diversity and abundance of soil fungal community varied significantly in the three cotton growing regions indicating soil type and environmental effects. These results suggest that changes in soil fungal community may play a notable role in soilborne disease incidence in cotton.

The importance of cotton disease surveys in Queensland for monitoring endemic diseases and detecting new pathogens and pests

The cotton industry in Australia funds biannual disease surveys conducted by plant pathologists. The objective of these surveys is to monitor the distribution and importance of key endemic pests and record the presence or absence of new or exotic diseases. Surveys have been conducted in Queensland since 2002/03, with surveillance undertaken by experienced plant pathologists. Monitoring of endemic diseases indicates the impact of farming practices on disease incidence and severity. The information collected gives direction to cotton disease research. Routine diagnostics has provided early detection of new disease problems which include 1) the identification of Nematospora coryli, a pathogenic yeast associated with seed and internal boll rot; and 2) Rotylenchulus reniformis, a plant-parasitic nematode. This finding established the need for an intensive survey of the Theodore district revealing that reniform was prevalent across the district at populations causing up to 30% yield loss. Surveys have identified an exotic defoliating strain (VCG 1A) and non-defoliating strains of Verticillium dahliae, which cause Verticillium wilt. An intensive study of the diversity of V. dahliae and the impact these strains have on cotton are underway. Results demonstrate the necessity of general multi-pest surveillance systems in broad acre agriculture in providing (1) an ongoing evaluation of current integrated disease management practices and (2) early detection for a suite of exotic pests and previously unknown pests.

Single gene resistance to Fusarium oxysporum f. sp. cubense Race 4 in the wild banana Musa acuminata subsp. malaccensis

Fusarium wilt of banana, caused by the fungal pathogen Fusarium oxysporum f. sp. cubense (Foc), is one of the most destructive diseases of banana. A particularly virulent strain of the pathogen, tropical race 4 (TR4), presents an emerging threat to banana producing regions throughout the world. No commercially acceptable banana cultivar is resistant to TR4 and, as with all strains of the Fusarium wilt pathogen, there is no effective chemical control. Genetic resistance to TR4 has been observed in the diploid wild banana Musa acuminata subsp. malaccensis, which has consequently received attention as a potential source of Fusarium resistance genes. The aim of this research was to determine the pattern of inheritance of the resistance trait by screening plants for resistance to Foc subtropical race 4 (SR4) and TR4. Our results showed that the F1 progeny of self-fertilized malaccensis plants challenged in pot trials against SR4 (VCGs 0120, 0129, 01211) and TR4 (VCG 01213/16) segregated for resistance according to a Mendelian ratio of 3:1 which is consistent with a single dominant gene hypothesis.

Collecting banana diversity in eastern Indonesia

Major diseases, including Fusarium wilt tropical race 4, threaten banana production systems worldwide. New sources of genetic resistance are considered necessary in the fight against such diseases. The triangular region of Indonesia taking in Sulawesi, the Maluku Islands and Lesser Sunda Islands was prioritized by the Global Musa Genetic Resources Network, MusaNet for exploration and collecting. It is just east of the Wallace Line, which is recognized as a transition zone for flora in southeast Asia, and had been little explored. Bioversity International funded a team of scientists from Indonesia and Australia to make collecting missions in the triangle in October 2012 and February 2013. Suckers and seeds of 35 promising new accessions were collected. About 90% of these are either wild species or diploid cultivars of more direct use to breeding programs. These were morphologically characterized during the collecting missions and included a set of photographs recommended by Bioversitys Taxonomic Advisory Group. Cigar leaf samples were also collected and sent as fresh samples to the International Banana Genotyping Centre in the Czech Republic. Ploidy and DNA (SSR) genotyping determinations from these samples have been invaluable in quickly interpreting and better appreciating what has been discovered. The new accessions have been grown on at Solok field collection, West Sumatra and will be made available by Indonesia to the international community, including breeding programs, for evaluation and utilization. Information on wild Eumusa prompts a rethinking of the phytogeography of Musa acuminata. The variation within the Australimusa species M. lolodensis highlights the need for broader study of this Musa section. French Plantain-like edible AAs and prospects for the generation of African plantains in the region were identified. The mission indicated existence of local edible ABs in eastern Indonesia in association with balbisiana hybrids origins in the region. Further explorations in the region should add to Musa diversity knowledge.