Mark E. Herrington

Rickettsia-like-organisms and phytoplasmas associated with diseases in Australian strawberries

Strawberry lethal yellows (SLY) disease in Australia is associated with the phytoplasmas Candidatus Phytoplasma australiense and tomato big bud, and a rickettsia-like-organism (RLO). Ca. P. australiense is also associated with strawberry green petal (SGP) disease. This study investigated the strength of the association of the different agents with SLY disease. We also documented the location of SLY or SGP plants, and measured whether they were RLO or phytoplasma positive. Symptomatic strawberry plants collected from south-east Queensland (Australia) between January 2000 and October 2002 were screened by PCR for both phytoplasmas and the RLO. Two previously unreported disease symptoms termed severe fruit distortion (SFD) and strawberry leaves from fruit (SLF) were observed during this study but there was no clear association between these symptoms and phytoplasmas or the RLO. Only two SGP diseased plants were observed and collected, compared with 363 plants with SLY disease symptoms. Of the 363 SLY samples, 117 tested positive for the RLO, 67 tested positive for Ca. P. australiense AGY strain and 11 plants tested positive for Ca. P. australiense PYL variant strain. On runner production farms at Stanthorpe, Queensland the RLO was detected in SLY diseased plants more frequently than for the phytoplasmas. On fruit production farms on the Sunshine Coast, Queensland, Ca. P. australiense was detected in SLY disease plants more frequently than the RLO.

Rain Damage on Three Strawberry Cultivars Grown in Subtropical Queensland

In Queensland, Australia, strawberry (Fragaria × ananassa Duchesne) is grown in open fields and rainfall events can damage fruit. Following a rain event, damage was evaluated on three commercial cultivars. ‘Rubygem’ (80%) had more fruit damaged than ‘Strawberry Festival’ (55%) and ‘Camarosa’ (61%). “Etch,” where the surface of the fruit is eroded and, consequently, the seeds are raised relative to the damaged surface, was the most frequently occurring (>80%) damage type and was distributed on the body and tip of the fruit, while some fruit (<16%) showed cracking. Fully mature fruit was damaged (>80%) more than partially mature fruit, which differed between ‘Strawberry Festival’ (16%) and ‘Rubygem’ (68%). Cultivars that are resistant to rain damage would reduce losses and lower risk for the growers.

‘Florida Beauty’ Strawberry

The leading strawberry (Fragaria ×ananassa) cultivar grown in Florida is currently ‘Florida Radiance’ (Chandler et al., 2009; U.S. Patent PP20,363) which was commercialized in 2009 and by the 2015–16 season, accounted for ≈70% of the acreage in west-central Florida. This cultivar is preferred for its early and total yields, its uniformly shaped and attractive fruit, and its open plant architecture and long pedicels which allow efficient harvesting. Sweet Sensation® ‘Florida127’ (Whitaker et al., 2015; hereafter referred to as ‘Florida127’; U.S. Patent PP25,574) is currently the second leading cultivar with ≈20% of acreage in 2016–17. This cultivar is preferred for its combination of high yields, large fruit size, excellent eating quality, and long shelf life. However, neither of these current commercial standards are well adapted for early transplanting dates in west-central Florida. High temperatures during transplanting tend to reduce early yields of ‘Florida Radiance’ and cause elongated fruit, which may be unmarketable, and ‘Florida127’ often becomes too vegetative when exposed to high temperatures. The planting window in west-central Florida has shifted earlier in recent years in an attempt to produce higher yields in late November and December when market prices are highest (Wu et al., 2015). There is currently a need for additional cultivars with uniformly shaped fruit and harvesting efficiency similar to ‘Florida Radiance’ and flavor characteristics similar to ‘Florida127’. There is also a need for cultivars that are adapted to early planting, between 25 Sept. and 1 Oct., when environmental conditions in west-central Florida are hot and unfavorable for flower initiation and ideal fruit shape. ‘Florida Beauty’ strawberry is the product of a collaborative breeding effort between the University of Florida and the Queensland Department of Agriculture and Fisheries. It has demonstrated adaptation to early planting, maintaining excellent fruit quality, and a yield distribution that complements current commercial standards. Experimental trials have been conducted under the direction of the University of Florida on the research plots of the Gulf Coast Research and Education Center (GCREC) (Balm, FL), at the Florida Strawberry Growers Association (FSGA) headquarters in Dover, FL, and on several farms in west-central Florida and Southern Spain.

Differences in pathogenicity, genetic variability and cultivar responses among isolates of Fusarium oxysporum from strawberry in Australia

Variation in the virulence of Fusarium oxysporum f. sp. fragariae (Fof) strains is important when evaluating the resistance of plants to this fungus. Twenty-five isolates of F. oxysporum harvested from strawberry (Fragaria ×ananassa) plants growing in Australia were characterized using pathogenicity tests, vegetative compatibility groups (VCGs), and genetic analysis of translation elongation factor 1 alpha (EF-1α). The level of disease varied depending on isolate used, indicating heterogeneous populations of Fof. Two distinct VCGs were identified and corresponded to two of the 10 lineages identified by partial EF-1α. Using a subset of Fof isolates, resistance in eight cultivars ranged from highly resistant to highly susceptible, with some cultivar × isolate interaction. ‘Strawberry Festival’, ‘QHI Sugarbaby’, and ‘DPI Rubygem’ had high levels of resistance across all isolates. Isolates from Western Australia (WA) were genetically distinct from those from Queensland (QLD) and were more virulent to ‘Camarosa’, a major cultivar grown in WA.