Blister spot is a disease of apple fruit caused by the bacterium Pseudomonas
syringae pv. papulans (Rose) Dhanvantari. The fruit of over 20
apple cultivars have been reported to be susceptible to blister spot (3) (Fig.
1). Blister spot also affects apple leaves, leaf petioles, and sometimes
terminal growth (Fig. 2). The foliar phase generally causes no economic damage
on mature trees but can be of considerable concern in nurseries where it may
restrict terminal growth. In 1998, several cultivars of apple were affected by
foliar and terminal infections in NY nurseries (T. J. Burr, unpublished).
Fortunately the disease is usually of economic importance only on the cultivar
‘Mutsu’ (also called ‘Crispin’) in Ohio and New York. In growing seasons
with wet weather during bloom and for several weeks after petal fall, nearly
100% of the fruit can be infected in Mutsu orchards. Although the
bacterium does not cause extensive decay of the fruit, it does induce lesions on
the fruit that make them unsuitable for fresh market use.
Studies by Burr et al. (2, 3, 4) reported on the epidemiology and control of blister spot. They showed that early infections occur through stomata on the fruit (2). Inoculation experiments revealed that fruit are most susceptible to infection from 2 to 2.5 weeks after petal fall and remain susceptible for an additional 4 to 5 weeks or until stomata on fruit developed into lenticels (3). Current control recommendations for blister spot consist of properly timed applications of streptomycin (8). The first spray is applied at 2 weeks after petal fall, with two additional applications made at weekly intervals. These recommendations were developed based on streptomycin spray timing trials conducted in New York (1), in which good disease control was obtained. When streptomycin was first used by Ohio growers in 1983, most growers obtained consistently good to excellent control of blister spot; even in years with weather highly conducive to disease development (M. A. Ellis, unpublished). In recent years, the level of control obtained from applications of streptomycin in Ohio has been variable. In years that are not highly conducive to disease development, nearly 100% control has been achieved by many growers; however, in seasons with excessively wet weather during the period of fruit susceptibility, losses of marketable Mutsu fruit due to blister spot often exceed 50% even when streptomycin applications are made (M. A. Ellis, unpublished). Improved methods for controlling this disease would be highly beneficial to growers of Mutsu apples.
In 1995, fosetyl-Al (Aliette 80WDG) [Aventis Crop Science, Research Triangle Park, NC] was registered for control of blister spot on apple. Studies in New York demonstrated that fosetyl-Al provides control of blister spot on apple (6, 7). The purpose of this study was to evaluate the efficacy of fosetyl-Al and streptomycin sulfate (Agri-mycin 17 WP) [Novartis Crop Protection Ag Products, Greensboro, NC] alone and in combination for control of blister spot of apple in Ohio and New York.
Field trials were established in a 10-yr-old commercial orchard of Mutsu apples on M.26 rootstock near Geneva, NY, in 1995, and in a 9-yr-old commercial orchard of Mutsu on M.7 rootstock near Wooster, OH, in 1996. In New York, treatments were replicated three times with five trees per replication in a completely randomized block design. In Ohio, treatments were replicated four times with one tree per replication in a completely randomized block design. Treated trees were separated by one untreated tree within the row to serve as a buffer. At both locations, trees were sprayed to runoff (approximately 3,741 liters of water per hectare) with a single nozzle hand gun at a pressure of 3,102 kPa. Treatments were timed according to current disease control recommendations for blister spot (8). In Ohio, fosetyl-Al at 4.48 kg a.i./ha, streptomycin at 382 g a.i./ha, and fosetyl-Al at 4.48 kg a.i./ha plus streptomycin 382 g a.i./ha were applied at two weeks after petal fall on 3 June. Additional applications of each treatment were applied on 11 and 18 June. In New York, the same treatments were applied on 8, 16, and 23 June. Disease severity was estimated for 50 randomly-selected fruit from each replication in early October using the following rating scale: 0-1 lesions/fruit (rating [R] = 1); 2-10 lesions/fruit (R = 2); 11-50 lesions/fruit (R = 3); and 50 or more lesions per fruit (R = 4). Fruit with an R value of 1 are acceptable for sale on the fresh market. Disease severity was estimated by converting the rating for each fruit into the mid-point of the severity interval (0.5, 6, 30 and 75 for R = 1, 2, 3, and 4, respectively) and calculating the mean for each replication and treatment. Data were analyzed with analysis of variance and means were separated with the Waller-Duncan K-ratio t-test (P = 0.05).
The experiments were repeated in Ohio in 1997, and in New York in 1996 as previously described. Treatments were applied to the same trees during both years in Ohio. In New York, treatments were applied within the same orchard, but on different trees in 1996. In Ohio, the first application was applied on 11 June, and two additional applications were made on 18 and 25 June. In New York, the first application was made on 23 May, followed by two additional applications on 30 May and 13 June.
Streptomycin resistance in P. syringae pv. papulans was first reported in 1988 (5). Both orchards in which these experiments were conducted had a history of streptomycin usage of at least six years. We suspected that the apparent reduction in blister spot control with streptomycin could be partially due to resistance development. In order to determine the sensitivity of P. syringae pv. papulans to streptomycin in the New York orchard, at least 10 fruit with blister spot were randomly collected from trees of all treatments within the experiment each year. Blister spot lesions were individually cut from each fruit (at least four per fruit) and were crushed in a drop of water on a microscope slide and then streaked on King’s medium B (9). In most cases pure isolates of P. syringae pv. papulans were obtained. Isolates produced smooth cream-colored colonies with a pale blue fluorescence. They also produced a negative response in the oxidase test (11). A water suspension (0.1 ml containing about 106 cfu/ml) of P. syringae pv. papulans from each isolate was uniformly spread on a plate of King’s medium B. Filter paper discs (3-mm diameter) were saturated with different concentrations of streptomycin sulfate and then placed on the plate. Each isolate was exposed to discs saturated with sterile distilled water, 10, 100, 1000 and 10,000 ug/ml streptomycin sulfate. Zones of growth inhibition around the discs were recorded after 24 hours incubation at 28 °C.
In October 1997, apple fruits with typical blister spot symptoms were collected from nonsprayed trees within the Ohio orchard. Four replicate isolations were made from individual lesions on four separate fruits (one fruit from each untreated control tree). The 16 isolates were tested for their sensitivity to streptomycin as previously described.
Disease severity during the two years of testing varied with location. We have observed that during relatively wet seasons the incidence and severity of blister spot is greatest. This probably results from the effect of free moisture spreading the bacterial cells in the orchard and possibly facilitating the penetration of the bacteria into the fruit. During both years of the study in Ohio, disease pressure was high. In untreated controls, the percentage of fruit suitable for fresh-market sale was only 20 and 18.5% in 1996 and 1997, respectively (Table 1). Abundant rainfall and moderate temperatures during the 5-week period from petal fall through 1 week after the third application (peak period of fruit susceptibility) provided conditions that were highly conducive to disease development. For the five 1-week periods beginning 20 May in 1996 (petal fall) and ending 25 June (1 week after the third application), the weekly total rainfall (in cm) and the average temperature (°C), respectively, were: 1.2 and 17; 1.6 and 13; 6.3 and 18; 3.6 and 21; and 1.0 and 23. In 1997, weekly rainfall and average temperature for the five 1-week periods from petal fall (29 May) through one week after the third spray were: 6.4 and 16; 1.9 and 18; 3.1 and 19, 0.8; and 24 and 0.1 and 22.
In 1995, conditions in the New York orchard were very dry for the period when fruit are most susceptible to blister spot. Only 2.5 mm of rain fell between 5 and 26 June. Therefore, 65% of untreated control fruit were acceptable for fresh market. In 1996, however, conditions were much more conducive to disease development, resulting in only 32% of the untreated controls being rated as acceptable for fresh market (Table 2). From 23 May through 13 June there were 5 rain events and a total of 5.6 cm of rainfall.
Treatment effects were similar for all experiments in Ohio and New York. In the 1996 and 1997 experiments in Ohio, respectively, 80 and 82% of the fruit on nontreated trees had more than one lesion per fruit and were considered unsuitable for fresh market use (Table 1). During both years of testing in Ohio, all treatments significantly increased the percentage of fresh market fruit (one lesion or less per fruit) over the untreated control. The combination of fosetyl-Al plus streptomycin resulted in significantly more marketable fruit than fosetyl-Al or streptomycin alone and there were no significant differences in percentage fresh market fruit between fosetyl-Al and streptomycin applied alone for both years of testing.
In 1996 in Ohio, all treatments significantly reduced disease severity (Table 1) compared to the control, and there were no significant differences in disease severity among the three treatments. In 1997, disease severity was again significantly lower in all treatments than in the untreated control (Table 1). The combination of fosetyl-Al plus streptomycin resulted in significantly less disease severity than any other treatment in 1997 and there were no significant differences in disease severity between fosetyl-Al and streptomycin.
In the New York experiments, all treatments resulted in a significantly greater percentage of fresh market fruit and significantly less disease severity than the untreated control in both years of testing, except for fosetyl-Al alone in 1996 (Table 2). As in the Ohio experiments, fosetyl-Al plus streptomycin applications always resulted in a higher percentage of marketable fruit and lower disease severity than streptomycin alone; however, the values were not significantly different at P = 0.05.
Although the level of disease control obtained from all treatments in the Ohio experiments was significantly better than the untreated control, the percentage of fresh market fruit from the most effective treatment (fosetyl-Al plus streptomycin) was only 51 and 39% in 1996 and 1997, respectively. Excessively wet weather during the peak period of fruit susceptibility during both years of testing may partially explain the relatively poor level of disease control obtained in these studies. Although current recommendations for timing spray applications were followed, improper timing may also partially explain poor disease control. Burr and Hurwitz (2) demonstrated that fruit susceptibility increases 2 to 2.5 weeks after petal fall and postulated that this period of susceptibility may vary during years when climatic conditions delay post petal fall fruit development. They further stated that the first application of streptomycin had to be made before the onset of increased fruit susceptibility (3). By following a strict calendar spray schedule, it is possible that sprays were not initiated early enough and the spray interval was too long.
As mentioned previously, the New York orchard had a long history of
streptomycin usage. Of 40 isolates of P. syringae pv. papulans
tested in 1996 from the orchard in New York, 32 were resistant to streptomycin.
All sensitive isolates formed no zones of inhibition around discs that were
saturated with water or 10 ppm streptomycin sulfate but formed zones at all
other concentrations. For resistant isolates, we did not observe differences in
the level of resistance. All were resistant to streptomycin at 10,000 ug/ml. Of
the 16 isolates obtained from fruit in Ohio, nine were resistant to
streptomycin. The presence of streptomycin-resistant isolates in the test
orchards may also partially explain the poor control obtained by streptomycin in
these tests. The presence of resistant isolates may also partially explain the
additive effect of fosetyl-Al plus streptomycin. The addition of fosetyl-Al to
streptomycin may have enhanced the control of blister spot in both orchards by
suppressing disease that may have been initiated by streptomycin-resistant
strains of the pathogen. The presence of streptomycin-resistant strains of the
pathogen would not explain the relatively poor control provided by fosetyl-Al
alone (Tables 1, 2). The reason behind the apparent difference in efficacy of
fosetyl-Al alone in 1995 and 1996 in the New York experiments is unknown.
However, subsequent studies have shown that acidic pH solutions of fosetyl-Al
are much more bactericidal than basic solutions, and therefore differences in pH
of water in the spray tanks may have affected efficacy (10).
Although blister spot is a serious problem on Mutsu apples, the acreage of Mutsu in Ohio is low; therefore, the disease is relatively unimportant for most apple growers. Mutsu has been an important cultivar in New York for several years. In the early 1980's, streptomycin effectively controlled the disease in both states; however, the development of streptomycin resistance has limited the usefulness of this antibiotic. Fosetyl-Al offers a possible alternative to streptomycin and may have an additive effect when combined with streptomycin. Although this spray combination provided significant disease control in our tests, it is an expensive treatment and did not provide a commercially acceptable level of disease control. Based on current price quotes, the cost for materials only of three applications of fosetyl-Al, streptomycin, and fosetyl-Al plus streptomycin at the rates used in this study would be approximately: $429/ha ($174/A); $283/ha ($115/A); and $712/ha ($288/A), respectively. The level of control obtained in these studies would not justify such costs. At present, fosetyl-Al is not recommended for control of blister spot in Ohio and New York. In addition, the use of streptomycin is not recommended in orchards were streptomycin-resistant strains of P. syringae pv. papulans are present.
If more economical and effective controls for blister spot cannot be found in the near future, it is likely that the acreage of Mutsu will decline. Of greater concern is that many of the cultivars currently planted throughout the eastern U.S. are susceptible to fruit infection (3); however, the disease rarely develops on these cultivars in the field. As new cultivars are introduced, blister spot could become a more serious disease. In 1996, blister spot was observed for the first time on ‘Fuji’ (Fig. 3) apples in three Ohio orchards and on ‘Sun Crisp’ (Fig. 4) apples in one Ohio orchard (M. A. Ellis, unpublished). Blister spot has also been observed on Fuji, ‘Golden Delicious’ (smoothie strain), and ‘Red Cort’ apples in Michigan (A. L. Jones, personal communication). These observations suggest that blister spot could represent a significant threat to commercial production of new blister spot-susceptible cultivars. The low level of control provided by currently registered bactericides in these tests suggest that additional research is needed to develop more effective control measures, or to improve the effectiveness of our current control measures for blister spot.
Salaries and research support provided by State and Federal funds appropriated to the Ohio Agricultural Research and Development Center, The Ohio State University, and the New York State Agricultural Experiment Station, Cornell University. We wish to thank the Ohio Fruit Grower’s Society, The Ohio Fruit Grower’s Marketing Association, and The New York Apple Research and Development Program for partial support of this research.
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11. Jones, A. L. 1971. Bacterial canker of sweet cherry in Michigan. Plant Dis. Reptr. 55:961-965.