© 2008 Plant Management Network.
Occurrence and Prevalence of Foliar Diseases on Blueberry in Georgia
H. Scherm, A. T. Savelle, and P. M. Brannen, Department of Plant Pathology, University of Georgia, Athens 30602; and G. Krewer, Department of Horticulture, University of Georgia, Tifton 31793
Scherm, H., Savelle, A. T., Brannen, P. M., and Krewer, G. 2007. Occurrence and prevalence of foliar diseases on blueberry in Georgia. Online. Plant Health Progress doi:10.1094/PHP-2008-0421-01-RS.
Qualitative and quantitative information about the occurrence and prevalence of fungal species causing foliar diseases on blueberry in the southeastern United States is limited. To close this knowledge gap, a field survey was conducted in 2002 and 2003 involving a total of 177 samples (cultivar-site-year combinations) from plantings of rabbiteye and southern highbush blueberry cultivars in Georgia. Among eight cultivars, Powderblue was the least affected cultivar whereas Star had high foliar disease levels in both years. Averaged by cultivar and year, there was a significant positive correlation between defoliation and disease severity. Septoria leaf spot and Gloeosporium leaf spot were the most prevalent and most severe foliar diseases, occurring on > 30% of samples in both years. Pestalotia leaf spot, Phyllosticta leaf spot, and leaf rust occurred at intermediate prevalence levels. Among these three diseases, leaf rust was most severe (especially in 2003) whereas Pestalotia leaf spot was associated with secondary infections and occurred mostly on southern highbush cultivars. Algal leaf spot and powdery mildew occurred infrequently and inconsistently. More than 70% of leaf samples, especially from the rabbiteye cultivars, showed symptoms of non-pathogenic leaf damage. The foliar disease complex on blueberry in Georgia is considerably more diverse than previously appreciated.
Blueberry production in Georgia has increased almost exponentially since the mid-1970s (31) to reach a statewide farm gate value of $75 million from 4,100 bearing hectares in 2006 (5). Based on the value of utilized production, Georgia ranks fourth among the blueberry-producing states in the nation. Most of the production area is located in the southeastern coastal flatwoods region and is planted to various cultivars of rabbiteye blueberry (Vaccinium virgatum = V. ashei), a native species that has been in cultivation for > 100 years (16). Within the past decade, an industry focusing on southern highbush blueberry (V. corymbosum interspecific hybrids) has emerged and is rapidly increasing in acreage and economic importance. Production of the latter species is more challenging, requiring well drained, higher organic matter soils and greater management inputs, but potential returns are higher due to an earlier harvest window and more favorable prices (31).
As production area and intensity have increased, disease problems have become more prevalent in Georgia blueberries. For example, foliar diseases collectively are considered "major" or "moderate" constraints by > 50% of blueberry growers, based on a survey of production problems in the southeastern United States (32). Indeed, recent yield loss estimates indicate that foliar diseases are responsible for up to 30% of the total disease-related blueberry losses in Georgia (38). Epidemics of foliar diseases have been shown to reduce carbon assimilation of affected plants (30) and lead to premature defoliation in the summer and fall, reduced flower bud set during the fall and winter, and lower return yields in the next year (6,8,25,26,28,29).
The Compendium of Blueberry Diseases (7) describes nine fungal taxa causing leaf spots, powdery mildew, or rust on blueberry in North America: Alternaria tenuissima (Alternaria leaf spot), Colletotrichum spp. (principally C. gloeosporioides causing anthracnose), Dothichiza caroliniana (double spot), Erysiphe vaccinii (=Microsphaera vaccinii causing powdery mildew), Gloeocercospora inconspicua (Gloeocercospora leaf spot), Gloeosporium minus (Gloeosporium leaf spot), Phyllosticta vaccinii (Phyllosticta leaf spot), Septoria albopunctata (Septoria leaf spot), and Thekopsora minima (=Pucciniastrum vaccinii causing leaf rust). Of these, only D. caroliniana, E. vaccinii, S. albopunctata, and T. minima have been documented formally in Georgia based on a search of the fungus-host data base maintained by the USDA-ARS Systematic Botany and Mycology Laboratory in Beltsville, MD (12). However, most of these records are from fungus or disease indices published in the 1960s (1,14), prior to the rapid expansion and intensification of the Georgia blueberry industry using improved rabbiteye and southern highbush blueberry cultivars (16). As such, qualitative and quantitative information about the occurrence of fungal species causing foliar diseases on blueberry in Georgia is limited. To close this knowledge gap, a 2-year field survey was initiated to determine the occurrence and prevalence of foliar diseases in rabbiteye and southern highbush blueberry plantings across the state.
Field Survey of Blueberry Plantings
The survey was conducted in late summer and fall (mostly between mid-August and late October) of 2002 and 2003 on a total of 16 farms (referred to as sites henceforth) in southern Georgia, five in northern Georgia, and one in upstate South Carolina near the Georgia border. Three of the sites were University of Georgia research plantings, whereas the remaining sites were commercial operations. The survey included rabbiteye cultivars Brightwell, Climax, Powderblue, Premier, and Tifblue, and southern highbush cultivars Star, O’Neal (2002 only), and Bluecrisp (2003 only). Each site was sampled once per season and included multiple cultivars, although not all cultivars were available for sampling at all sites.
At each site, percent defoliation was determined in the field based on the number of leaves missing from nodes on 20 arbitrarily selected spring shoots from 20 bushes per cultivar. Disease prevalence (based on presence or absence of a particular disease on at least 5% of the leaves in a given sample) was determined either in the field on the same 20 shoots, or on 200 to 400 leaves collected from the field and taken to the laboratory. Disease severity (percent necrotic leaf area) was estimated visually on a subsample of 50 to 100 leaves. The assessor conducting the laboratory assessments was trained with DiseasePro (24), a computerized disease assessment training program, prior to evaluating disease severity. The identity of all pathogens associated with foliar diseases was determined based on symptoms and characteristic signs such as type of asexual fruiting structure, presence or absence of setae, and conidial morphology (Appendix). Where multiple foliar diseases were present, the identity of the primary disease (i.e., that contributing most to disease severity within that sample) was noted. Overall, the survey included 78 and 99 samples in 2002 and 2003, respectively.
Data analyses included descriptive statistics such as mean values of disease severity and defoliation as well as prevalence of individual diseases across all samples and separately by cultivar. Correlation analysis was used to determine the association between defoliation and disease severity across cultivars. All analyses were conducted using SAS v. 9.1 (SAS Institute Inc., Cary, NC).
A Diverse Foliar Disease Complex on Blueberry
Disease severity. Averaged across all samples, percent necrotic leaf area was lower in 2002 (7.8%) than in 2003 (19.2%). This was likely due to less favorable weather for disease development during the summer and early fall of 2002 when large parts of Georgia were abnormally to excessively dry (2). In contrast, frequent summer rains in 2003 resulted in favorably to abnormally moist conditions across the state (3). For Septoria leaf spot, Roloff et al. (30) documented that a disease severity of 20%, similar to the average level of necrotic leaf area in our survey in 2003, reduced net assimilation of affected blueberry leaves by ~50%. Although the quantitative relationship between photosynthesis and disease severity is likely to vary among blueberry leaf pathogens, it is clear that the disease levels observed in our survey are of a magnitude sufficient to have a major adverse impact on carbohydrate supply during a critical time period when the flower buds that provide the foundation for next year’s yield are initiated (10,11).
Foliar disease severity varied among and within cultivars (Fig. 1, Table 1). On average, Powderblue was the least affected cultivar in both years, but the genotypes with the highest disease severity levels differed between years (Star in 2002 and Climax in 2003). This inconsistent ranking of cultivars between the 2 years was most likely weather-related, with dry conditions in 2002 and wet conditions in 2003 favoring different pathogens to which the cultivars responded differentially. For example, Climax, which had the second-lowest disease severity in 2002, was affected severely by leaf rust in 2003 (see below), causing it to be ranked highest in average disease levels that year.
Defoliation. Defoliation at the time of assessment, averaged across all samples, was lower in 2002 (11.8%) than in 2003 (25.1%); this was consistent with the differences in disease severity between the 2 years. The ranking of cultivars relative to defoliation mirrored that of disease severity (Table 1), resulting in a significant positive correlation between defoliation and disease severity across cultivars (r = 0.691, P = 0.0062, n = 14). A significant association between premature defoliation and percent necrotic leaf area has been described previously for Septoria leaf spot (25,29), and our observations here extend this relationship to foliar diseases of blueberry in general. Premature fall defoliation is well documented to reduce flower bud set and yield potential for both rabbiteye and southern highbush blueberry (17,37).
Septoria and Gloeosporium leaf spots. Averaged across cultivars, Septoria leaf spot (Fig. 2A and B) and Gloeosporium leaf spot (Fig. 2C and D) were the most prevalent foliar diseases occurring on > 30% of samples in both years (Fig. 3). In addition to having the highest prevalence, these two leaf spots also were recorded most commonly as being the primary (most severe) foliar diseases on samples: Septoria in about half of the cases and Gloeosporium in about a quarter of the cases (Fig. 4). Among the cultivars included in the survey, Septoria leaf spot occurred most frequently on Star, Brightwell, and Bluecrisp and with the lowest frequency on O’Neal and Climax (Table 1). Patterns were less clear-cut for Gloeosporium leaf spot, where most cultivars fell within an intermediate to high prevalence range.
Given the high prevalence and severity of Septoria and Gloeosporium leaf spots, research to develop resistant cultivars and evaluate cultural and chemical management tactics should focus on these two diseases. Unfortunately, as far as the authors are aware, resistance against foliar diseases in general and Septoria and Gloeosporium leaf spots in particular is not currently a high priority in blueberry breeding programs in the Southeast (18,23). However, a considerable body of epidemiological information has been generated for Septoria leaf spot to serve as a basis for improved fungicidal management of the disease (25,26,27,28,29). In contrast, information on the epidemiology of Gloeosporium leaf spot is more limited (19,35), pointing to an important future research need.
In this context it is important to note the tenuous taxonomic status of G. minus, the causal agent of Gloeosporium leaf spot. Most members of the form-genus Gloeosporium have been transferred to other coelomycete or ascomycete genera during the past 50 years (36). However, G. minus so far has escaped closer scrutiny and taxonomic revision, hence Shear’s (34) original name is retained here. To complicate matters further, foliar anthracnose-type symptoms similar to those of Gloeosporium leaf spot may be caused by C. gloeosporioides (20). The latter species may be distinguished from G. minus based on the production of salmon-colored (vs. hyaline) spore masses in predominantly setose (vs. glabrous) acervuli. In our survey, foliar anthracnose caused by C. gloeosporioides was encountered on only three leaf samples, all from Star southern highbush blueberry. Because of its infrequent occurrence, foliar anthracnose caused by C. gloeosporioides is not discussed in detail here. However, as the acreage planted to Star continues to increase in Georgia, further surveys to determine the relative frequencies of anthracnose vs. Gloeosporium leaf spot would be warranted.
Pestalotia leaf spot. Another anthracnose-type leaf spot, caused by a Pestalotia sp. (Fig. 2E), was the third most common foliar disease, occurring on about 25% of the samples in both years of the survey (Fig. 3). The disease was consistently more common on southern highbush than on rabbiteye cultivars (Table 1). In many cases, symptoms and signs were associated with secondary infections of leaf areas affected previously by Gloeosporium leaf spot. Indeed, despite its common occurrence, Pestalotia leaf spot was recorded only infrequently (in < 4% of cases) as the primary foliar disease in our survey (Fig. 4). This is consistent with the fact that Pestalotia spp. generally are considered weak or secondary pathogens (13). Several species of this fungus have been reported previously from leaves of wild and cultivated Vaccinium species, including P. vaccinii on cranberry (V. macrocarpon) in Michigan (33) and P. planimi and P. vaccinicola on sparkleberry (V. arboreum) in Florida (13). The identity and pathogenicity of the Pestalotia species on leaves of cultivated blueberry in Georgia remains to be determined.
Leaf rust and Phyllosticta leaf spot. Leaf rust (Fig. 2F) and Phyllosticta leaf spot (Fig. 2G and H) occurred at intermediate frequencies in our survey (Fig. 3). However, leaf rust generally was more severe (Fig. 4), having been recorded as the primary foliar disease in almost 20% of the samples in 2003, the wet year. For both diseases, prevalence and severity appeared to be strongly cultivar-dependent. For example, in 2003, leaf rust was considerably more common on Climax and Bluecrisp than on the other cultivars (Table 1). In contrast, Phyllosticta was absent from Star in both years. To what extent these observations reflect true differences in cultivar susceptibility vs. disease escape needs to be examined in future research. As with most other foliar diseases of blueberry, information about epidemiology and management of leaf rust and Phyllosticta leaf spot is very limited (7). Based on our observations, leaf rust generally occurs earlier than the other foliar diseases and may require fungicide applications before the fruit are harvested in early summer. In contrast, fungicide applications before fruit harvest in early summer have tended to have little effect on later-occurring diseases such as Septoria leaf spot (6).
Algal leaf spot and powdery mildew. Algal leaf spot (Fig. 2I) and powdery mildew (Fig. 2J) occurred infrequently and inconsistently, i.e., in only one of the 2 years (Fig. 3) and on only some cultivars (Table 1). In addition, the two diseases were not severe, having been recorded as the primary disease on < 3% of samples (Fig. 4). The algal species associated with leaf spotting was identified as Cephaleuros virescens, a common epiphyte and parasite on perennial plants in the southeastern United States, including blueberry (22). In general, algal leaf spot is favored by wet weather (as in 2003) and occurs more commonly on weakened plants.
Although reported previously from Georgia (4), double spot, a foliar disease caused by D. caroliniana, was not encountered in our study. The disease has been observed only on highbush (21) and southern highbush (4) blueberry, so its absence from the rabbiteye cultivars in our survey should not come as a surprise. The previous report of double spot from Georgia was on a US 41×G-362 southern highbush cross in a germplasm evaluation test in Tifton, GA (4). The southern highbush cultivars included in our survey may be resistant to double spot, or the disease may not be widespread in the state.
Non-pathogenic leaf damage. More than 70% of leaf samples in both years, especially from the rabbiteye cultivars, showed symptoms (sometimes severe) of damage that could not be associated with a specific pathogen. Symptoms ranged from edema-like watersoaking (Fig. 2K) and necrotic blotching (Fig. 2L), likely caused by heat or other physiological stresses (15), to irregular, sunken, necrotic areas with damaged epidermis on the lower leaf surface (Fig. 2M); the latter symptom may have been due to mechanical damage (possibly blowing sand) or arthropod feeding. In a comparative physiological study, Roloff et al. (30) determined that both pathogen-induced necrosis and edema-like leaf damage reduced net assimilation of blueberry leaves, although the relative magnitude of the reduction was less pronounced on edema-affected leaves for a similar size of necrotic leaf area. In addition, the virtual lesion size of edema-affected leaves, a measure of the physiological effects of the damage on the remaining healthy green leaf tissue, was about one-third lower than that of Septoria leaf spot.
This study showed that the foliar disease complex on rabbiteye and highbush blueberry in Georgia is considerably more diverse than previously appreciated. In fact, 52.5 and 19.2% of leaf samples had, respectively, at least two and three diseases present simultaneously. Septoria leaf spot and Gloeosporium leaf spot were the most prevalent and most severe foliar diseases, followed by Pestalotia and Phyllosticta leaf spots in prevalence and leaf rust in severity. On average, samples with higher disease severity showed increased levels of defoliation. Some tentative differences in cultivar susceptibility were noted, with Powderblue having the lowest foliar diseases severity overall, Climax least affected by Septoria leaf spot but susceptible to leaf rust, and Phyllosticta leaf spot absent from Star in both years. Based on this research and the authors’ subsequent observations from 2004 to 2007, the occurrence of specific foliar diseases on certain blueberry cultivars in Georgia has been consistent. Septoria leaf spot is an important problem on Star and Brightwell, Gloeosporium occurs commonly across most cultivars, and leaf rust and Phyllosticta leaf spot are recurrent on Climax and Bluecrisp. On these susceptible cultivars and in conditions conducive to disease development, summer and early-fall fungicide applications against foliar diseases are now recommended in Georgia and neighboring states (9).
Given the high prevalence and severity of Septoria and Gloeosporium leaf spots, research to develop resistant cultivars and evaluate cultural and chemical management tactics should focus on these two diseases. In addition, the frequent occurrence of non-pathogenic leaf damage indicates a need to focus on identifying potentially damaging arthropod pests, improving plant nutritional status, and avoiding water stress, particularly after fruit harvest during the hottest part of the season in summer and early fall.
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