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Barbara J. Smith

Three Colletotrichum species—Colletotrichum acutatum J.H. Simmonds (teleomorph Glomerella acutata J.C. Guerber & J.C. Correll), Colletotrichum fragariae A.N. Brooks, and Colletotrichum gloeosporioides (Penz.) Penz. & Sacc. in Penz. [teleomorph Glomerella cingulata (Stoneman) Spauld. & H. Schrenk]—are major pathogens of strawberry (Fragaria ×ananassa). Strawberry anthracnose crown rot has been a destructive disease in commercial strawberry fields in the southeastern United States since the 1930s. The causal fungus, C. fragariae, may infect all aboveground plant parts; however, the disease is most severe when the fungus infects the crown, causing crown rot, wilt, and death. Colletotrichum gloeosporioides was responsible for an epidemic of anthracnose crown rot in strawberry nurseries in Arkansas and North Carolina in the late 1970s. The anthracnose fruit rot pathogen, C. acutatum, was first reported in 1986 on strawberry in the United States. Since the 1980s, increased losses due to anthracnose fruit and crown rots in the United States may be related to changes in cultivars and to widespread use of annual plasticulture production rather than the matted-row production system. Anthracnose investigations in the United States have concentrated on its epidemiology and differences among the three causal Colletotrichum spp. in their cultural, morphological, and molecular characteristics; their infection processes; and their pathogenicity. Results from these studies have resulted in a better understanding of the diseases and have led to better disease control. Strawberries grown in soils with high nitrogen levels are more susceptible to anthracnose than are those grown in soils with lower nitrogen levels or those amended with calcium nitrate. Anthracnose is spread more rapidly in fields that have overhead irrigation and plastic mulch than in fields where drip irrigation and straw mulch are used. Fungicide efficacy has been determined in in-vitro, greenhouse, and field studies, and pathogen resistance to some fungicides has been detected. Anthracnose-resistant cultivars are a major objective of most strawberry breeding programs in the southern United States.

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Creighton L. Gupton and Barbara J. Smith

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James R. Ballington and Barbara J. Smith

Thirty-three accessions of Fragaria virginiana collected from Mississippi in 1995 were evaluated for horticultural traits and leaf disease resistance at Reidsville, N.C., and strawberry anthracnose resistance (Colletotrichum acutatum and C. fragariae) at Poplarville, Miss., in 1997. The range of variability in berry shape, fruit flesh color, fruit skin toughness, and degree of sunkenness of seeds among accessions indicated probable introgression with F. xananassa in most all accessions. Seventeen of 29 accessions screened for resistance to C. acutatum were resistant, and an additional 10 were tolerant. Overall, these accessions appear to be good additional sources of resistance to this, the prevalent species of anthracnose in the southeastern United States. In addition, the majority of accessions appear to be tolerant-resistant to leaf scorch, leaf blight, and/or powdery mildew. Nine accessions were resistant to all three leaf diseases, and four were resistant to C. acutatum as well as the three foliar diseases. No accessions were resistant to C. fragariae and only five were tolerant. All five accessions tolerant to C. fragariae were also either resistant or tolerant to C. acutatum but the converse was not true.

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Creighton L. Gupton and Barbara J. Smith

Eight cultivars, including five recent releases, five selections from the Florida AES, and 16 selections from the Georgia AES were planted in the muscadine germplasm working collection at McNeil, Miss., in 1992. All cultivars and one replication of the selections were evaluated in 1997. None of the new cultivars yielded as much as `Fry', the standard fresh fruit cultivar. The percent dry picking scar of `Dixie' and `Fry' was low. `Tara', `Polyanna', and `Fry' produced the largest berries. Percent soluble solids was lowest in `Fry', `Nesbitt', and `Alachua' but highest in `Dixie' berries. `Fry', `Alachua', and `Polyanna' had the lowest and the other cultivars did not differ in number of seed per berry. One selection, 33-1-4, appeared to have the qualities of a potential cultivar. Incidence and severity of berry rots were generally low.

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Creighton L. Gupton and Barbara J. Smith

A study was conducted to determine if blackberry cultivars Humble, Rosborough, and Brazos transmit rosette (incited by Cercosporella rubi) resistance and to estimate heritability (h2). Plants of parents and offspring involving these cultivars were rated for rosette severity on a whole plant basis from 0 = no rosette to 7 = all buds infected or plant dead. An estimate of h2 was computed by regression of cross means on parental means. The mean rosette severity rating of plants from crosses was always intermediate between the cultivar and other parents. Only `Humble' transmitted enough rosette tolerance to be usable although `Rosborough' crosses were more tolerant than the other parents. The h2 estimate of 0.48 was fairly high but low variability among parents other than `Humble' would suggest little progress from mass selection.

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Creighton L. Gupton and Barbara J. Smith

Experiments were conducted to estimate the relative importance of additive and dominance genetic variances and non-allelic interactions in the inheritance of resistance to Colletotrichum spp. in strawberry (Fragaria × ananassa Duch.). Progeny of 40 parents crossed in a Comstock and Robinson Design II Mating scheme were inoculated with three isolates of C. fragariae and one isolate of C. acutatum. Disease development on each plant was rated visually. Variance components were estimated and converted to genetic variances. Estimates of were six to 10 times higher than those for Within-family variance not accounted for by equaled 35% and 38% of the total genetic variance in females and males, respectively, indicating probable epistatic effects. The frequency distribution of disease severity ratings was bimodal in both experiments, suggesting major gene action. Narrow-sense heritability estimates were 0.37 and 0.26, and broad-sense heritability estimates were 0.87 and 0.85 for females and males, respectively. Narrow-sense heritability estimates are probably sufficient to produce gains from recurrent selection. Gains from selection of clonal value should be possible because of the high broad sense heritability estimates. It appears feasible to establish a broad genetic-based population resistant to Colletotrichum spp. from which selections could be evaluated per se and/or recombined to produce improved populations.

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John L. Maas, Gene J. Galletta and Barbara J. Smith

We have determined in tests conducted both at Beltsville and Poplarville that several strawberry isolates of Colletotrichum acutatum, C. gloeosporioides and C. fragariae produce toxin-like compounds in culture. Crude culture filtrates (CFI elicited general and specific responses in tomato and strawberry plants. Tomato plants initially were used because they are highly responsive to toxins in general, whereas the reaction of strawberry plants apparently is greatly affected by environmental and nutritional growing conditions of the test plant. Toxin symptoms included leaf chlorosis and wilting, leaf midvein darkening, and plant death when CF was applied to leaves or if seedlings or petioles were immersed into CF. Juvenile tissues appear to be more susceptible to the effects of the toxins than mature tissue. No differences in response to culture filtrates were apparent among those from the Colletotrichum isolates. The putative toxins appear to act differentially with susceptible or resistant strawberry germplasm.

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William C. Olien, Barbara J. Smith and C. Patrick Hegwood Jr.

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Mary Helen Ferguson, Christopher A. Clark and Barbara J. Smith

Xylella fastidiosa Wells et al. causes disease in a number of plants in the southeastern United States, including southern highbush blueberry (Vaccinium corymbosum interspecific hybrids), but little was known concerning its potential impact in rabbiteye blueberry (Vaccinium virgatum Aiton syn. Vaccinium ashei Reade). In a naturally infected orchard in Louisiana, mean yields of X. fastidiosa–positive plants were 55% and 62% less than those of X. fastidiosa–negative plants in 2013 and 2014, respectively. Average berry weight was also lower in X. fastidiosa–positive plants. Within 3 years of testing positive for X. fastidiosa, four of nine X. fastidiosa–positive plants appeared dead. However, plants that were X. fastidiosa–negative in 2013 remained so until 2015, indicating that the bacterium did not spread rapidly in this established orchard during this time. Other factors, including soil chemistry variables, Phytophthora cinnamomi, ring nematode, and ringspot symptoms, were also investigated to determine if one of these might predispose plants to infection with X. fastidiosa or be partly responsible for observed yield loss. In most cases, interactions were not found, but associations with soil Cu and Zn suggest a need for further research on whether these elements predispose rabbiteye blueberry to X. fastidiosa infection and thereby contribute to yield losses. Researchers, extension workers, and growers should be aware of X. fastidiosa as a potential yield- and survival-impacting factor in rabbiteye blueberry.

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Ebrahiem M. Babiker, Stephen J. Stringer, Hamidou F. Sakhanokho, Barbara J. Smith and James J. Polashock

Species of Botryosphaeria and Neofusicoccum are major pathogens of blueberry worldwide. Accurate identification of these species is essential for developing effective management practices. A multigene sequencing strategy was used to distinguish between six isolates of stem blight pathogens collected from two different regions of the United States. The temperature growth study revealed that the optimal temperature for growth of five of the tested isolates ranged from 25 to 30 °C, although no significant difference was detected for the growth of Neofusicoccum spp. isolate SD16-86 at 20, 25, 30, and 35 °C. In vitro fungicide assays showed four fungicides, cyprodinil + fludioxonil, propiconazole, pyraclostrobin + boscalid, and azoxystrobin, were effective against the tested isolates with isolate SD16-86 being less sensitive compared with the other isolates. In a detached stem assay, none of 39 blueberry accessions displayed immunity or a high level of resistance to the two tested isolates, and no significant difference in lesion length was detected among the seven tested Vaccinium species inoculated with the two isolates.