Phytophthora infestans has been a named pathogen for well over 150 years and yet it continues to "emerge", with thousands of articles published each year on it and the late blight disease that it ...causes. This review explores five attributes of this oomycete pathogen that maintain this constant attention. First, the historical tragedy associated with this disease (Irish potato famine) causes many people to be fascinated with the pathogen. Current technology now enables investigators to answer some questions of historical significance. Second, the devastation caused by the pathogen continues to appear in surprising new locations or with surprising new intensity. Third, populations of P. infestans worldwide are in flux, with changes that have major implications to disease management. Fourth, the genomics revolution has enabled investigators to make tremendous progress in terms of understanding the molecular biology (especially the pathogenicity) of P. infestans. Fifth, there remain many compelling unanswered questions.
A new disease of potatoes, tentatively named zebra chip (ZC) because of the intermittent dark and light symptom pattern in affected tubers which is enhanced by frying, was first found in Mexico in ...1994 and in the southwestern United States in 2000. The disease can cause severe economic losses in all market classes of potatoes. The cause of ZC has been elusive, and only recently has been associated with 'Candidatus Liberibacter' sp. Field samples of potato plants were collected from several locations in the United States, Mexico, and Guatemala to determine transmission to potato and tomato by grafting of ZC-infected scions and psyllid feeding. The disease was successfully transmitted, through up to three generations, by sequential top- and side-grafting ZC-infection scions to several potato cultivars and to tomato. The disease was also successfully transmitted to potato and tomato plants in greenhouse experiments by potato psyllids collected from potato plants naturally affected with ZC. Transmission electron microscopic observation of ZC-affected tissues revealed the presence of bacteria-like organisms (BLOs) in the phloem of potato and tomato plants inoculated by grafting and psyllid feeding. The BLOs were morphologically similar in appearance to BLOs associated with other plant diseases. Polymerase chain reaction (PCR) amplified 16S rDNA sequences from samples representing different geographic areas, including the United States, Mexico, and Guatemala, were almost identical to the 16S rDNA of 'Ca. L. solanacearum' previously reported from solanaceous plants in New Zealand and the United States. Two subclades were identified that differed in two single base-pair substitutions. New specific primers along with an innovative rapid PCR were developed. This test allows the detection of the bacteria in less than 90 min. These data confirm the association of 'Ca. L. solanacearum' with potatoes affected by ZC in the United States, Mexico, and Guatemala.
The tomato late blight pandemic of 2009 made late blight into a household term in much of the eastern United States. Many home gardeners and many organic producers lost most if not all of their ...tomato crop, and their experiences were reported in the mainstream press. Some CSAs (Community Supported Agriculture) could not provide tomatoes to their members. In response, many questions emerged: How did it happen? What was unusual about this event compared to previous late blight epidemics? What is the current situation in 2012 and what can be done? It's easiest to answer these questions, and to understand the recent epidemics of late blight, if one knows a bit of the history of the disease and the biology of the causal agent, Phytophthora infestans.
The COVID-19 pandemic altered the way many consumers and businesses transacted business. Concerning the green industry, many households began gardening and/or purchased more green industry products. ...As the pandemic ends and households begin to return to normal, green industry firms need to understand this new normal. Using an online national survey of households, we assessed which households were more likely to remain in the market after entering during the height of the pandemic (2020). Findings indicated that younger consumers (i.e., Millennials and younger individuals who were born in 1985 or after) were less likely to indicate they always garden (before the pandemic) but more likely to have started gardening during the pandemic and perceived that they would not continue to garden as states returned to normal (2021). This age group was also more likely to not have gardened in 2020, but they intended to garden in 2021. This finding shows a dichotomy in gardening preferences in this young age group. Further findings indicated that race, household income, number of children in the household, and the impact of the pandemic on the household also help explain the household’s decision to garden or not.
Fusarium spp. are responsible for significant yield losses in sugar beet (Beta vulgaris) with Fusarium oxysporum f. sp. betae most often reported as the primary causal agent. Recently, a new species, ...F. secorum, was reported to cause disease in sugar beet but little is known on the range of virulence within F. secorum or how this compares to the virulence and phylogenetic relationships previously reported for Fusarium pathogens of sugar beet. To initiate this study, partial translation elongation factor 1‐α (TEF1) sequences from seven isolates of F. secorum were obtained and the data were added to a previously published phylogenetic tree that includes F. oxysporum f. sp. betae. Unexpectedly, the F. secorum strains nested into a distinct group that included isolates previously reported as F. oxysporum f. sp. betae. These results prompted an expanded phylogenetic analysis of TEF1 sequences from genomes of publicly available Fusarium spp., resulting in the additional discovery that some isolates previously reported as F. oxysporum f. sp. betae are F. commune, a species that is not known to be a sugar beet pathogen. Inoculation of sugar beet with differing genetic backgrounds demonstrated that all Fusarium strains have a significant range in virulence depending on cultivar. Taken together, the data suggest that F. secorum is more widespread than previously thought. Consequently, future screening for disease resistance should rely on isolates representing the full diversity of the Fusarium population that impacts sugar beet.
A 4-year study (1997 to 2000) was conducted to determine the sensitivities of the potato tuber rot pathogens, Phytophthora erythroseptica and Pythium ultimum, to mefenoxam (metalaxyl). A total of ...2,277 tubers showing symptoms of "water rot" were collected from 16 states and 2 Canadian provinces. From these, 849 isolates of P. erythroseptica and 213 isolates of P. ultimum were obtained, and 805 and 190 isolates, respectively, were tested for their ability to grow on V8 medium amended at 0.01 to 100 μg/ml with fungicide. Isolates ranged widely in their responses to mefenoxam. The presence of resistant isolates (EC
> 100 μg ml
) of P. erythroseptica in the potato producing areas of Maine was confirmed. The presence of P. erythroseptica isolates in Idaho and Minnesota resistant to mefenoxam is reported for the first time. The proportion of P. erythroseptica isolates resistant to mefenoxam varied from 2.9 to 36.2% between 1997 and 2000. The proportion of resistant P. ultimum isolates represented only a small proportion of the isolates tested (3.7%). A single resistant P. ultimum isolate was recovered from Washington, whereas most of the resistant isolates obtained (5 of 7) were collected in Minnesota during the final year of the study. This is the first report of resistance in P. ultimum pathogenic to potato tubers. These observations suggest that pink rot and leak could become significant problems in the future, particularly in those areas where resistance has been detected. Our results have implications for the effective management of water rot. Monitoring the sensitivity of the pathogen population to mefenoxam in all production areas should be considered and integrated as a part of the overall disease management strategy.
Gibberella zeae, a causal agent of Fusarium head blight (FHB) in wheat and barley, is one of the most economically harmful pathogens of cereals in the United States. In recent years, the known host ...range of G. zeae has also expanded to noncereal crops. However, there is a lack of information on the population genetic structure of G. zeae associated with noncereal crops and across wheat cultivars. To test the hypothesis that G. zeae populations sampled from barley, wheat, potato, and sugar beet in the Upper Midwest of the United States are not mixtures of species or G. zeae clades, we analyzed sequence data of G. zeae, and confirmed that all populations studied were present in the same clade of G. zeae. Ten variable number tandem repeat (VNTR) markers were used to determine the genetic structure of G. zeae from the four crop populations. To examine the effect of wheat cultivars on the pathogen populations, 227 strains were sampled from 10 subpopulations according to wheat cultivar types. The VNTR markers also were used to analyze the genetic structure of these subpopulations. In all populations, gene (H = 0.453 to 0.612) and genotype diversity (GD = >0.984) were high. There was little or no indication of linkage disequilibrium (LD) in all G. zeae populations and subpopulations. In addition, high gene flow (Nm) values were observed between cereal and noncereal populations (Nm = 10.69) and between FHB resistant and susceptible wheat cultivar subpopulations (Nm = 16.072), suggesting low population differentiation of G. zeae in this region. Analysis of molecular variance also revealed high genetic variation (>80%) among individuals within populations and subpopulations. However, low genetic variation (<5%) was observed between cereal and noncereal populations and between resistant and susceptible wheat subpopulations. Overall, these results suggest that the populations or subpopulations are likely a single large population of G. zeae affecting crops in the upper Midwest of the United States.
A 2004-2005 survey of potatoes from stores in the north‐central potato‐producing region of the USA showed that the predominant causes of dry rot were Fusarium graminearum and Fusarium sambucinum. ...Isolates of F. graminearum originally isolated from potato tubers with dry rot (n = 15), wheat kernels with scab (n = 15), and sugarbeet tap roots with decay (n = 5) were tested for aggressiveness to potato tubers. There were no significant differences in aggressiveness among isolates of F. graminearum, regardless of original host, as measured by their ability to cause dry rot. These findings may have implications for survival of F. graminearum inoculum since potatoes, wheat and sugarbeets are frequently used in crop rotation in the region. Fusarium graminearum required larger wounds for infection of potato tubers than F. sambucinum. Plug‐removal injury, simulating a stolon‐removal injury, resulted in equal incidence of dry rot caused by the two Fusarium species, whereas abrasion and bruising injury were sufficient for infection and dry rot development by F. sambucinum, but not F. graminearum. A change in harvest practices from vine‐killing prior to harvest to mechanical vine‐killing on the day of harvest may be a factor affecting the onset of dry rot caused by F. graminearum, since this process often causes large wounds at the stem end of the tubers when the stolon is forcibly removed.
In all, 98 isolates of three Fusarium spp. (18 Fusarium oxysporum, 30 F. graminearum, and 50 Fusarium sp. nov.) obtained from sugar beet in Minnesota were characterized for pathogenicity and ...virulence on sugar beet in the greenhouse by a bare-root inoculation method. Among the 98 isolates tested, 80% of isolates were pathogenic: 83% of the F. oxysporum isolates, 57% of the F. graminearum isolates, and 92% of the Fusarium sp. nov. isolates. Symptoms varied from slight to moderate wilting of the foliage, interveinal chlorosis and necrosis, and vascular discoloration of the taproot without any external root symptoms. Among the pathogenic isolates, 14% were highly virulent and 12% were moderately virulent. Most of the highly virulent isolates (91%) and moderately virulent isolates (89%) were Fusarium sp. nov. All pathogenic isolates of F. graminearum and most pathogenic isolates (87%) of F. oxysporum were less virulent. In general, more-virulent isolates induced first foliar symptoms earlier compared with less-virulent isolates. This study indicates that both F. oxysporum and Fusarium sp. nov. should be used in greenhouse and be present in field studies used for screening and developing sugar beet cultivars resistant to Fusarium yellows complex for Minnesota and North Dakota.
Cercospora beticola survives as stromata in infected crop residue. Spores produced on these survival structures serve as primary inoculum during the next cropping season. This study was conducted to ...determine how long C. beticola can survive at different soil depths, the mechanism of inoculum dispersal, and the primary infection site in sugar beet. Longevity of C. beticola was studied over a 3-year period under field conditions at Fargo, ND. C. beticola-infected leaves were placed at depths of 0, 10, and 20 cm and retrieved after 10, 22, and 34 months. Survival of C. beticola inoculum declined with time and soil depth. Inoculum left on the soil surface, 0 cm in depth, survived the longest (22 months) compared with that buried at 10 cm (10 months) and 20 cm (10 months). C. beticola dispersal from the primary source of inoculum was studied in the field for three growing seasons. Sugar beet plants were surrounded with plastic cages with and without ground cover, or exposed with and without ground cover. Significantly higher disease severity was observed on exposed plants than caged plants with or without ground cover, suggesting that wind was the major dispersal factor for C. beticola inoculum. The primary infection site by C. beticola was determined in a greenhouse study. Leaves, roots, and stems of healthy sugar beet plants were inoculated with C. beticola. Cercospora leaf spot symptoms were observed only on plants that were leaf inoculated, suggesting that the leaf was the primary infection site for C. beticola.
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