2. Fungal diversity

‘Fungal diversity’ considers three species that illustrate the spectacular range of structural complexity found among fungi: Olpidium brassicae, Spirodactylon aureum, and Sphaerobolus stellatus. There are more than 70,000 species of fungi described by mycologists and over 90% of them are classified within Phylum Basidiomycota (basidiomycetes) and Phylum Ascomycota (ascomycetes). Half of the basidiomycetes produce mushrooms; the others include rusts and smuts that cause plant disease. The ascomycetes include the yeast Saccharomyces cerevisiae, fungi with beautiful cup-shaped fruit bodies, truffles, and morels. The other major groups of fungi are less well known and include species whose cells swim in water.

Specific amino acids of Olive mild mosaic virus coat protein are involved in transmission by Olpidium brassicae

Transmission of Olive mild mosaic virus (OMMV) is facilitated by Olpidium brassicae (Wor.) Dang. An OMMV mutant (OMMVL11) containing two changes in the coat protein (CP), asparagine to tyrosine at position 189 and alanine to threonine at position 216, has been shown not to be Olpidium brassicae-transmissible owing to inefficient attachment of virions to zoospores. In this study, these amino acid changes were separately introduced into the OMMV genome through site-directed mutagenesis, and the asparagine-to-tyrosine change was shown to be largely responsible for the loss of transmission. Analysis of the structure of OMMV CP by comparative modelling approaches showed that this change is located in the interior of the virus particle and the alanine-to-threonine change is exposed on the surface. The asparagine-to-tyrosine change may indirectly affect attachment via changes in the conformation of viral CP subunits, altering the receptor binding site and thus preventing binding to the fungal zoospore.

Introgression of Novel Alleles for Partial Resistance to Big Vein Disease from Lactuca virosa into Cultivated Lettuce

Big vein is an economically damaging disease of lettuce (Lactuca sativa L.) incited by Mirafiori lettuce big vein virus, which is vectored by the soil-borne fungus Olpidium brassicae (Woronin) P.A. Dang. Resistance to this disease is needed because no feasible cultural control methods have been identified. Partial resistance is available within cultivated lettuce and is expressed as delayed appearance of symptoms in combination with a reduced percentage of symptomatic plants. Complete resistance has been identified only in accessions of L. virosa L., an incongruent wild relative of lettuce. Resistance from L. virosa has not been introgressed into lettuce. The objective of this research was to determine whether big vein resistance from L. virosa can be introgressed into lettuce. Progenies of backcross (BC) hybrids between L. virosa and L. sativa cultivars were greenhouse tested for big vein resistance over four generations of self-pollination. Selected plants from resistant BC families were used as parents to create BC2 progeny from crosses with high partial-resistant cultivars, intermediate partial-resistant cultivars, and susceptible cultivars to test for the presence of transgressive segregants. Experiments were conducted in the greenhouse by infecting seedlings with O. brassicae zoospores collected from big vein symptomatic plants. Plots were evaluated for area under the disease progress curve and the percentage of symptomatic plants; asymptomatic plants from resistant families were retained in every generation. Complete resistance to big vein was not recovered, and may be the result of insufficient sampling of BCF2 progeny or linkage between resistance alleles and alleles causing incongruity. Variation for partial resistance was observed in all BC generations, and transgressive segregants were identified among BC2 families from crosses using partially resistant and susceptible parents. This research suggests that L. virosa contains alleles that confer partial resistance to big vein when introgressed into an L. sativa background, and these alleles are distinct from those present in partially resistant lettuce cultivars. Alternative breeding strategies should be pursued to introgress complete resistance from L. virosa into cultivated lettuce.

A Transgenic Lettuce Line with Resistance to Both Lettuce Big-vein Associated Virus and Mirafiori Lettuce Virus

The coat protein (CP) gene of lettuce big-vein associated virus (LBVaV) in sense or antisense orientation in a binary vector pBI121 was transformed via Agrobacterium tumefaciens (Smith and Towns.) Conn. mediated transformation into lettuce (Lactuca sativa L.) to generate LBVaV-resistant lettuce. Nineteen T1 lines were produced; five to 10 plants of each T1 line were inoculated with LBVaV using Olpidium brassicae (Wor.) Dang.; and LBVaV was not detected in eight plants derived from six lines. T2 seedlings from the eight plants were tested for LBVaV resistance, and one line (line A-2) with the CP gene in antisense orientation was resistant to LBVaV while the other lines were susceptible. The transgenic line A-2 was also resistant to mirafiori lettuce virus (MiLV) and big-vein expressions regardless of the presence or absence of LBVaV.

Host Resistance to Mirafiori lettuce big-vein virus and Lettuce big-vein associated virus and Virus Sequence Diversity and Frequency in California

Big vein is an economically damaging disease of lettuce (Lactuca sativa) caused by the Olpidium brassicae-vectored Mirafiori lettuce big-vein virus (MLBVV). Lettuce big-vein associated virus (LBVaV) is also frequently identified in symptomatic plants, but no causal relationship has been demonstrated. Although big vein is a perennial problem in the United States, the extent of MLBVV and LBVaV infection and diversity is unknown. Lettuce cultivars partially resistant to big vein reduce losses, but do not eliminate disease. While Lactuca virosa does not develop big vein symptoms, it has not been tested for infection with MLBVV or LBVaV. Lettuce cultivars Great Lakes 65, Pavane, Margarita, and L. virosa accession IVT280 were evaluated for big vein incidence and virus infection in inoculated greenhouse trials. Additional lettuce samples were collected from field sites in California, classified for symptom severity, and evaluated for virus infection. Reverse transcription-polymerase chain reaction and nucleotide sequencing were used to determine infection with MLBVV and LBVaV, and sequence diversity among viral isolates, respectively. Infections with MLBVV and MLBVV/LBVaV were dependent on big vein symptom expression in California production areas, and isolates were closely related to those found in Europe and Japan. Partial big vein resistance was identified in Margarita and Pavane; however, MLBVV infection was found in asymptomatic plants. L. virosa IVT280 remained symptomless and virus free, suggesting that it is immune to MLBVV and LBVaV.

Virus surveys of lettuce crops and management of lettuce bigvein disease in New Zealand

Virus surveys of lettuce crops over the past three seasons have confirmed that a number of virus diseases can threaten production Lettuce bigvein disease (LBVD) caused by Mirafiori lettuce bigvein virus (MLBVV) usually in combination with Lettuce bigvein virus (LBVV) was the most widespread virus disease of lettuce over the survey period Other viruses present include Lettuce necrotic yellows virus (LNYV) Beet western yellows virus (BWYV) Cucumber mosaic virus (CMV) and Lettuce mosaic virus (LMV) The surveys have not detected Tomato spotted wilt virus (TSWV) or Tobacco necrosis virus (TNV) Control of Olpidium brassicae the fungal vector of LBVD is an important factor in disease management This paper outlines survey results and describes experiments using fungicides to control this disease