1Present Address: Pioneer Hi-Bred International Inc., Johnston, IA 50131
Flyspeck (FS) and sooty blotch (SB) diseases of apple and pear are distributed widely in warm, moist areas of the world (13). Although distinct diseases, they frequently occur together and are often considered as the flyspeck/sooty blotch complex (FS/SB). In the USA, FS/SB is consistently economically important in the southern and mid-Atlantic states and often important elsewhere, especially on late-maturing cultivars (7). Though the diseases do not decrease yield, conspicuous presence of the causal fungi on the fruit surface can render fresh-market produce unappealing to consumers, with consequent economic loss to growers.
The fungus Schizothyrium pomi (Mont. & Fr.) v. Arx (asexual form, Zygophiala jamaicensis E. Mason) causes FS (13). SB is a disease complex induced by at least three fungi: Peltaster fructicola Johnson, Sutton & Hodges; Leptodontium elatius (G. Mangenot) de Hoog; and Geastrumia polystigmatis Batista & M.L. Farr (4,5,13). The FS/SB pathogens are superficial and grow on the surface of the fruit cuticle. SB colonies appear as diffuse, irregular, cloudy or sooty blotches varying in size and color from olive to brown (Fig. 1). Signs of the disease are attributable to various fungal structures (7,13). In contrast, FS appears as small (150 to 375 um diameter), black, shiny dots in clusters of a few to about 100 (7,13) (Fig. 2). The FS/SB pathogens over-winter on apple twigs and fruit, as well as on numerous other woody plants. Infection occurs in the spring or early summer following the spread of conidia or ascospores from reservoir hosts to developing fruit. Secondary infections produced by conidia originating from woody plants near the orchard occur throughout the growing season (13).
Although cultural practices such as pruning and sanitation reduce disease pressure, the primary means of controlling FS/SB is by fungicides (7,10,13). Normally, FS/SB is controlled in the upper Midwest, along with other secondary fungal diseases, as a side effect of fungicide programs directed primarily at the apple scab pathogen. However, where these sprays are reduced substantially or eliminated, as in orchards planted to scab-resistant cultivars, FS/SB can become a serious problem (8). Although sulfur is not specifically registered for FS/SB control, growers producing organic apples can use different formulations of sulfur as an acceptable general-use fungicide and miticide (2). Therefore we included it as a positive control against which to compare our treatments. To provide alternatives for commercial growers or home gardeners wishing to avoid or minimize the use of conventional fungicides, the efficacy of several compounds was explored over the past 3 years. Among these were formulations of methionine plus riboflavin with adjuvants (MR), and potassium bicarbonate combined with an oil polymer. These compounds were selected because they have been effective for control of powdery mildews and other fungal pathogens (3,9,11,12,14). We report here that summer sprays of MR or potassium bicarbonate give good control of FS/SB on scab-resistant apple cultivars in Wisconsin.
Four treatments were compared: (i) d, l-methionine (1 mM; Sigma, St Louis, MO) plus riboflavin (26.6 µM; Sigma), freshly mixed and supplemented with sodium dodecyl sulfate (SDS, 1 mg per mL; Sigma) and trace metal ions provided as 1 mM copper sulfate (CuSO4.5H2O; Sigma), a mixture referred to by us and others (11,12) as MR formulation; (ii) 0.5%, wt/vol, potassium bicarbonate (KHCO3; Sigma) with paraffin oil polymer (0.5%, vol/vol, SunSpray Ultra-Fine Oil; Sunoco, Philadelphia, PA) (14); (iii) wettable sulfur (80% AI Microfine WP; 357 mg/L; Platte Chemical Co., Freemont, NE) as a positive control; (iv) water as a negative control. Because there is abundant information (e.g., 14) that bicarbonates perform better with oil than without it, we did not include separate treatments of bicarbonate and oil alone. Likewise, because MR formulation, which includes the CuSO4 and SDS, is superior to methionine plus riboflavin without the adjuvants (12), we did not evaluate the components separately or in various permutations. Expressing CuSO4 in copper fungicides on a mM basis shows that the concentrations in commercial products are approximately 20 to 100 times greater than the 1 mM adjuvant in the MR formulation. Nevertheless, the possibility that this level, or the SDS, may be directly toxic should not be overlooked.
Experiments were conducted in commercial orchards at Cottage Grove and Gays Mills, WI. The former is 1.6 ha of 14-year-old trees on M.7 rootstock, spaced approximately 5 x 5 m. The latter is approximately 57 ha comprising 22 cultivars on M.7 rootstock, planted in 1989 and spaced about 6 x 5 m. This research program began in 1998 with applications to the scab-resistant cultivar Prima at Gays Mills. In 1999, trials were expanded to include Freedom and Jonafree at Gays Mills. In 2000, the experiments included Freedom and Jonafree at Gays Mills, as well as Jonafree at Cottage Grove.
The research was performed as branch trials in which randomly assigned scaffold branches approximately equidistant apart within the trees received the treatments. All developing fruit and surrounding leaves on the selected branches were treated. All treatments were represented once within each tree and were replicated six times across trees, i.e. arranged as randomized complete blocks with trees as the blocks. Treatments were applied with manually pressurized backpack sprayers (H. D. Hudson Mfg. Co, Chicago, IL) equipped with multi-spray nozzles as standard equipment adjusted to a conical spray pattern. Spraying commenced about 10 days after the petal fall stage each year: In 1998 this was on 3 June; in 1999 on 2 June; and in 2000 on 2 and 7 June at Gays Mills and Cottage Grove, respectively. Applications were made at weekly intervals in June, and thereafter biweekly, terminating on 11 August in 1998 and during the last week in August in 1999 and 2000. Inter-branch interference was avoided by the presence of untreated adjacent branches and application of treatments early or late in the day when drift was minimal.
FS/SB disease ratings were taken on 30 fruit per replicate branch in late August or early September. FS incidence (i.e., presence or absence) was rated and the number of diseased fruit expressed as a percentage of the total examined. SB severity was rated on a scale from 0 to 3 with 0 = no disease; 1 = trace (< 1% fruit surface affected); 2 = 1 to 5% surface affected; and 3 = >5% fruit surface affected. A 1 to 5 scheme used by raters on a trial basis in 1999 was converted on a linear scale to 0 to 3 to facilitate comparisons across years. This simple linear scale assumed disease differed by an equivalent amount between adjacent numerical classes, which is not strictly true. However, the analyses and conclusions did not change when we tested, on a trial basis, rating disease as a direct percentage of fruit surface infected as assessed by image analysis of fruit lesions.
In 2000, the field experiment was supplemented by applying treatments to apple fruit incubated in moist chambers. Immature apples (cv. Freedom) harvested from the Gays Mills orchard on 19 July were assigned randomly in groups of 32 to one of five treatments (the four field treatments identified above plus MR in darkness). Each treatment was replicated five times. Fruit were dipped in the compound, drained, and place on a wire mesh shelf above water in closed, clear plastic boxes. The containers were incubated in a growth chamber at 24ºC, under incandescent/fluorescent lighting with a 12-hour photoperiod. The MR darkness permutation was accomplished by wrapping the boxes in aluminum foil. Disease ratings were taken on 24 August according to the criteria described above for field studies.
Observations and Comments
Sulfur significantly reduced FS on all cultivars every year (Figs. 3 to 5). Sulfur was not as consistently effective against SB, failing to reduce this disease significantly in three of the seven trials. More interestingly, in every year and on every cultivar, MR significantly reduced both FS and SB as well as or better than sulfur. Potassium bicarbonate was effective against both FS and SB, though it did not perform as consistently as MR.
MR and potassium bicarbonate also performed as well as sulfur against FS under controlled conditions in a growth chamber (Fig. 6). SB did not develop under these conditions and therefore was not rated. MR was as efficacious under constant darkness as under alternating dark-light conditions, which was unexpected because sunlight or artificial light is thought to promote generation of oxygen radicals, the postulated mode of action of MR (11,12). However, more recent evidence suggests that riboflavin may act by activating disease resistance genes in the host (1). Potassium bicarbonate is apparently both fungistatic and fungicidal (9), but its cellular mode of action was not determined and is not clear from the literature (e.g., 9,14). Elsewhere (14), bicarbonate with a film-forming polymer has proven effective against foliar diseases of cucurbits; bicarbonate salts added to water agar have inhibited growth of the cucurbit (14) and other (9) pathogens.
MR and potassium bicarbonate treatments significantly reduce FS/SB disease under Wisconsin conditions. This finding warrants further consideration as an alternative to fungicides for growers who cannot or will not apply conventional chemicals, or as an adjunct to IPM programs. We are expanding our trials to test the treatments under yet more diverse conditions and investigating the mode of action and potential side effects, such as phytotoxicity, of these compounds.
We thank Nick Voichick, Rick Voland, John Edmunds, and many student hourly helpers for assistance with fieldwork, and Russ Spear for help with the figures and statistical analyses of the data. This research was supported by the CSREES, USDA, and the Nebraska Agricultural Experiment Station-Lincoln, under Cooperative Agreement no. 99-COOP-1-7686.
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