Our main objective is to develop a biologically based program to control the major fungal diseases of apple fruit and foliage in the northern United States. If successful, this would break the continuing reliance on toxic fungicides used by conventional growers and provide a reliable, non-pesticide, low input, and low cost framework for organic growers.
The specific objectives of our current research are to:
1. Assess the ability of an orchard floor cover crop, kura clover, to break the life cycle of the apple scab pathogen.
2. Test the efficacy of environmentally benign compounds used as sprays during the growing season to control scab and other important foliar and fruit diseases, such as cedar apple rust and the sooty blotch/flyspeck complex.
Inoculum (ascospores) of the apple scab pathogen is released in the spring from previously infected apple leaves that have overwintered on the orchard floor. Kura clover is proposed to reduce ascospore dispersal by both providing a barrier to prevent the escape of ascospores from the clover canopy as well as promoting decomposition of the apple leaf litter. For Objective 1, apple plots at two locations were seeded with kura clover in spring 1997. In spring 1998 and 1999, ascospore release was measured using spore samplers positioned above the kura clover plots and control plots (existing ground cover). Apple scab disease was also monitored on the leaves of the orchard trees and leaves of small potted trees placed in the orchard as bioassay plants. Earthworm activity was assessed by counting castings in the kura and control plots. For Objective 2, three environmentally benign experimental sprays, benzothiadiazole ([BTH], an inducer of disease resistance), a methionine (amino acid)-riboflavin (vitamin) mixture (M-R), and potassium bicarbonate (KHCO3), were applied to apple tree branches. Apple scab, cedar-apple rust and the flyspeck/sooty blotch disease complex on apple leaves and fruit were assessed throughout the season.
In 1998, kura clover significantly reduced aerial ascospore concentration compared to the existing ground cover control at one location but not at the other. It did not reduce apple scab disease at either location. In 1999, kura clover did not significantly reduce the aerial ascospore concentration compared to the existing ground cover at either location, nor did it significantly reduce apple scab symptoms on the leaves of the mature or potted bioassay trees at either location. There were significantly more earthworm castings and holes at one location but not at the other.
In 1998 all of the experimental sprays reduced apple scab infection on leaves and fruit, under mild to moderate disease pressure. In 1999, under moderate to severe disease pressure, M-R and BTH reduced apple scab on both leaves and fruit. M-R and KHCO3 significantly reduced sooty blotch and flyspeck infection on fruit in both years. Cedar apple rust infection on leaves or fruit was not evaluated in 1999, because results from 1998 showed little impact of the compounds on this disease.
Impacts and Potential Contributions
Kura clover takes at least three years to become well developed. At the site where the clover is better established, earthworm activity is greater and there is a trend toward lower scab incidence on potted trees. A decrease in aerial ascospores and scab is expected when a better stand of the clover is established. Identifying environmentally benign alternatives to synthetic fungicides will especially benefit organic growers but will also relieve traditional orchardists from dependence on more toxic compounds. Combining the clover as a ground cover with the use of the most effective of the experimental sprays should result in effective, non-toxic, and sustainable disease control.
Kura clover was seeded as a ground cover at two apple orchards in spring 1997 to test its ability to decrease aerial ascospore concentrations of Venturia inaequalis and thus inhibit primary apple scab infection. In 1999, at the Peninsular Research Station in Sturgeon Bay, WI, ascospore counts in kura plots were not different from counts in a control plot with a grass ground cover. The ascospore counts were too low at a second site, the West Madison Research Station in Madison, WI, for a meaningful analysis. At neither location did the kura clover result in a decrease in primary scab infection on the leaves of the orchard trees or potted bioassay trees placed in the orchard. However, kura clover requires approximately three years to develop a dense stand, and we anticipate that the clover will prove to be more effective at preventing primary scab infection in future years.
Three experimental sprays, benzothiadiazole ([BTH], an inducer of systemic acquired resistance), a methionine-riboflavin mixture (M-R), and a potassium bicarbonate-Ultrafine Sunspray Oil™ mixture (KHCO3), as well as a water control, were applied to apple trees at Sturgeon Bay and at Turkey Ridge organic orchard near Gays Mills, WI. Two treatments (BTH and M-R) reduced apple scab on leaves and fruit; two treatments (M-R and KHCO3) reduced sooty blotch and flyspeck symptoms on fruit. Cedar apple rust infection on leaves or fruit was not evaluated in 1999, because results from 1998 showed little impact of the compounds on this disease.
1. Test the ability of the orchard floor cover crop kura clover (Trifolium ambiguum) to break the life cycle of the apple scab pathogen (Venturia inaequalis).
2. Test the efficacy of environmentally benign compounds as sprays during the growing season to control scab and other important foliar and fruit diseases such as cedar apple rust and the sooty blotch/flyspeck complex.
Objective 1. Orchard Floor Treatments
a. Plot establishment
In anticipation of possible funding by SARE, we seeded kura clover in two orchards in spring 1997, before the project started. Two 90 ft x 240 ft (27 m x 73 m) plots, each containing approximately 80 trees (cultivar/rootstock: Empire/M.26 in plot 1; Redmax/M.26 in plot 2) were established at the U.W. Peninsular Research Station, Sturgeon Bay, WI (site 1). One 60 ft x 160 ft (18 m x 48 m) plot containing 24, 15-year-old trees was established at the U.W. West Madison Agricultural Station, Madison, WI (site 2). Floor treatments were 1) seeded kura clover; or 2) the orchard floor maintained as existing ground cover (EGC), primarily grass. Each treatment was performed twice at Sturgeon Bay and once at Madison. Kura clover (Treatment 1) was seeded at 8 kg/ha and inoculated with commercial rhizobia (Liphatech, Inc.), following standard site preparation recommendations. Existing ground cover (EGC, Treatment 2), consisting of mixed grasses and weeds, was left unaltered. The focus in 1997-1999 was to promote rapid establishment of the kura clover and collect preliminary data (summers of 1998-1999).
b. Ascospore Quantification.
In 1999 rotorod spore samplers were placed 40 cm above the ground in the center of Treatment 1 (kura) and Treatment 2 (EGC) subplots at Madison and Sturgeon Bay on 9 April and 15 April, respectively. In 1999 we used two samplers (designated “inner” and “outer”) in each block separated by about 10m. The samplers at Madison were activated manually during periods of rain when ascospores were anticipated to be released during the primary scab infection cycle (3 April, green tip; 15 May petal fall) between 9 April – 25 May 1999. At Sturgeon Bay, the traps were activated electronically by leaf wetness sensors during the primary scab cycle, which occurred there between 14 April (green tip) and 21 May 1999 (petal fall). The first ascospores captured were on 18 April at West Farm and 23 April at Sturgeon Bay.
c. Disease Quantification.
Apple scab incidence (presence or absence) resulting from presumed primary infection was assessed on orchard trees at both sites on 7 June (Madison) and 8 June 1999 (Sturgeon Bay). Fifty terminal leaves and 50 spur leaves on each of three branches of six (Madison) or eight (Sturgeon Bay) trees per groundcover treatment were rated. Additionally, as bioassays for scab, healthy potted apple trees were placed in the kura and EGC subplots (eight trees per subplot) at both sites for 19-33 days during the primary cycle, returned to cold frames distant from obvious sources of inoculum, and incubated for 3-21 days, after which disease severity (numbers of lesions per leaf) was assessed.
d. Earthworm Activity.
Earthworm activity was assessed at both West Madison and Sturgeon Bay on 5 August and 11 August, respectively. Following removal of vegetation on six 76-cm diameter subplots, worm middens (castings) were counted. It should be noted that this technique is more an indicator of earthworm activity than an estimation of population numbers.
Objective 2. Treatments Applied to Trees
a. Treatment Application
The hypothesis being tested is that non-synthetic, environmentally benign organic compounds give commercially acceptable disease suppression. Treatments were applied at two locations: Whole-tree treatments in a 24-tree block of scab-susceptible Marshall McIntosh at the U.W. Peninsular Research Station at Sturgeon Bay; and branch treatments on 18 trees (six each of the scab-resistant varieties Freedom, Jona-Free, and Prima) at Turkey Ridge Organic Orchard, Gays Mills, WI.
Six repetitions of each of the following treatments were applied: i) methionine (1mM)-riboflavin (26.6µg/L)-copper sulfate (1mM)-sodium dodecyl sulfate (1000µg/mL); ii) potassium bicarbonate (0.5% weight/volume)-Ultrafine Sunspray Oil (5mL/L); iii) benzothiadiazole (BTH) (0.28g/gallon); iv) water (negative control); v) sulfur (positive control, Gays Mills only) (13.5g/gallon); vi) Nova/Dithane (commercial fungicide) at labeled rates (positive control, Sturgeon Bay only).
The methionine-riboflavin and the potassium bicarbonate treatments are considered fungal eradicants and were thus applied after the disease was visible. In 1999, sprays were applied at both locations, beginning on 26 May at Sturgeon Bay and on 2 June at Turkey Ridge. The sprays were applied weekly during June and bi-weekly during July and early August ending on 5 August and 4 August respectively at Sturgeon Bay and Turkey Ridge. BTH is an inducer of systemic acquired resistance and is thought to trigger the plant’s inherent disease resistance mechanism. In 1999 it was applied four times at weekly intervals starting on 5 May at Sturgeon Bay and 14 May at Turkey Ridge. The control treatments (water, sulfur, and Nova/Dithane) were sprayed during each application period at both locations.
b. Disease Quantification
Apple scab incidence was rated twice on the leaves (8 June and 29 June) and once on the fruit (16 September) of the whole-tree treatments at the Sturgeon Bay orchard. At Gays Mills, 30 fruit from each of six replicate trees of Prima, Jona-Free, and Freedom were harvested on 2 September, stored at 5C for one week and then rated for flyspeck incidence and sooty blotch severity. Flyspeck was rated as – / + for each fruit with a combined flyspeck colony size of 10% disease.
Objective 1. Orchard Floor Treatments
a. Ascospore Quantification
There were no significant differences in ascospores trapped on Rotorod samplers placed in kura clover and EGC ground treatments at Sturgeon Bay (Fig.1 and 2). At Madison, ascospore counts were too low for meaningful data analysis.
b. Disease Quantification
i) Orchard Trees
Numbers of primary scab lesions on the leaves of the orchard trees at the Sturgeon Bay site did not differ significantly between the kura clover and EGC treatments (Fig. 3). Similar results were seen at the Madison site (Fig. 4) although disease incidence was lower.
ii) Potted Bioassay Trees
Numbers of primary scab lesions on the leaves of the potted bioassay trees did not differ significantly between the kura clover and EGC treatments at both sites (Figs. 5 & 6). Potted trees that had been place in orchard plots were significantly more diseased than control trees placed in a cold frame.
A two-year-old stand of kura clover did not control the primary scab infection at either Madison or Sturgeon Bay. Kura clover requires approximately three years to out-compete weeds and to develop a dense stand.
d. Earthworm activity.
Earthworm activity (Fig. 7) was greater in the kura ground cover than EGC at West Madison. The same trend occurred at Sturgeon Bay but the difference was not as significant. This might be explained by drought conditions at the Sturgeon Bay site prior to sampling.
Objective 2. Treatments Applied to Trees
a. Apple Scab
Apple scab lesions were assessed in June on all leaves of five terminal shoots and five spurs from each of the six replicates of each treatment from the Sturgeon Bay site. Under moderate disease pressure conditions, the M-R and BTH treatments significantly reduced foliar scab infection compared to the water control (Fig. 8). Two treatments, M-R and BTH, significantly reduced scab infection on fruit compared to the water control (Fig. 9).
b. Cedar Apple Rust
No evaluations were made in 1999 for cedar apple rust.
c. Sooty Blotch/Flyspeck Complex
M-R reduced sooty blotch and flyspeck on the three varieties tested (Figs. 10 and 11). KHCO3 reduced sooty blotch on all three varieties and flyspeck on two varieties (Fig. 11). In some cases, M-R and KHCO3 were more effective than the sulfur control. BTH was not effective in controlling either sooty blotch or flyspeck.
The results of the three experimental sprays were promising for controlling fungal diseases on apple leaves and fruit. The M-R and BTH treatments significantly reduced apple scab on both fruit and leaves of the highly susceptible McIntosh variety under moderate to severe disease pressure. Two of the three experimental compounds (M-R and KHCO3) resulted in significant control of the flyspeck/sooty blotch complex. These data for 1999 confirm results reported previously for 1998.
Apple is consistently among the top five plant-based commodities in the United States and, in terms of commercial production (about 11 billion pounds valued at approximately $1 billion), second only to oranges among tree fruits. Internationally, as a producer, the United States ranks behind only China.
There are shortcomings in the conventional approaches for control of the production-limiting diseases such as apple scab. Despite various non-chemical options, including the planting of disease-resistant cultivars, the mainstay of control remains fungicide. This is expensive and has a deleterious environmental impact. Agrochemicals also pose a threat of adverse effects to human health arising by direct exposure during application (exacerbated by “urban sprawl” into agricultural areas) and indirectly by consumption of residues in food. About 90% of all fungicides used in agriculture are animal oncogens. Although fungicides constitute only about 10% of all pesticides applied to food crops, they account for roughly 75% of the oncogenic risk associated with consumption of processed foods. Because of concerns of carcinogenic effects, ethylenebisdithiocarbamate (EBDC) fungicides have been restricted for use on apples and other crops. Although additional fungicides are currently available, they are succumbing to resistant fungi or are undergoing intense review.
Our preliminary results (1998 data) indicated that kura clover as a groundcover reduces the primary inoculum of apple scab, although this result was not repeated in 1999. A reduction in primary inoculum should result in a decrease in primary infection. In future years when a better stand of the kura clover is achieved, we anticipate that a decrease in apple scab infection will follow. This reduction in scab infection should result in less reliance on fungicides, which are commonly applied 11-15 times per year (amounting to, e.g. 6 lbs. of Captan 50 WP per acre per application). Perhaps the traditional sprays could be reduced or eliminated altogether and replaced by a non-toxic material(s). In 1999, two of the three experimental compounds in this study reduced scab infection and two controlled sooty blotch and flyspeck. Organic farmers do not have the option of using traditional fungicide sprays. Thus, the development of a groundcover that controls scab, as well as effective non-toxic compounds is particularly beneficial to them. Likewise, the health of all consumers of apples would benefit from the reduction or elimination of traditional fungicides.
In adopting a ground cover of kura clover, initial costs include buying seed as well as labor inputs to prepare the soil, plant seed, and eliminate weeds. After a few years, these costs would diminish, as the kura clover out-competes the weeds. At that point merely mowing the orchard two to three times per year (less frequently than a grass ground cover) would be necessary.
The experimental sprays used in this study are currently more expensive than traditional fungicides ($0.25 to $0.30 per gallon vs. $0.08 per gallon for Captan 50 WP). However, if the chemicals prove to be effective alternatives to fungicides, presumably they could be purchased in bulk at substantial savings. (For our research, chemicals were purchased in small quantities from scientific suppliers.) Also, additional experimental sprays such as low cost compost sprays may be incorporated into the study.
Reducing apple scab or other fungal diseases will result in economic gains through higher quality fruit. Apples are typically graded by quality; a #1 or #2 apple is sold as fresh fruit whereas inferior apples are usually made into cider or vinegar. The grade is dependent somewhat on size and shape but primarily on surface blemishes such as disease lesions. At Turkey Ridge Organic Orchard, value decreases by 60% from a #1 apple to a cider apple ($30/box vs. $12/box). The experimental sprays tested in this study were shown to reduce the number of diseased apples by more than 50% which represents a potential 30% increase in profit for an apple grower.
No apple growers have begun using kura clover as a ground cover, though many have become familiar with its potential use during courses and field days (listed under 5. Publications/Outreach). Growers are inquiring about the availability of kura clover seed. However, we have not encouraged them to plant kura yet, because our data have not yet shown clear scab control benefits. Because the research is still in the early stages, only one grower has thus far been involved in our alternative spray experiment.
Educational & Outreach Activities
4 June 1997: IPM Field Day, Mequon, WI — 40 growers
7 Jan 1998: UW-Madison Apple Short Course, Madison, WI — 25 growers and students
7 May 1998: UW Institute for Pest and Pathogen Management, Madison, WI — 10 scientists
19 June 1998: Apple IPM Field Day, Sturgeon Bay, WI — 25 growers
6 Aug 1998: Disease Diagnosis and Control Field Course, Sturgeon Bay, WI — 10 students
23 Oct 1998: UW-Madison Plant Pathology Seminar, Madison, WI — 30 faculty and students
26 July 1999: Apple IPM Field Day, Hixton, WI — 50 growers
The Peninsular Research Station in Sturgeon Bay is on a major highway used by tourists. As such, the station receives many visitors. The kura/EGC plots for our research are included on the self-guided tour map.
We continually seek input from our grower collaborator, Mr. Bob Johnson of Turkey Ridge Organic Orchard, and other commercial growers. Their comments have already influenced our research. For example, they raised questions about bees pollinating kura clover (after apple bloom) being harmed by insecticides applied to apple trees. Thus, we are recording clover phenology and development in relation to apple tree phenology. They have also encouraged us to document the impact of kura clover on soil nutrients and rodent populations. We propose to address these issues in future studies.
Areas needing additional study
Orchard Floor Treatments
There is a need to assess the effect of the kura clover ground cover on the decomposition of the apple leaf litter. This is being achieved by collecting and weighing the apple leaf litter in the kura clover and existing ground cover plots.
Pertinent information would be gained by studying the non-target impacts of the kura clover on the soil system. Assessing: (i) earthworm counts; (ii) soil temperature/moisture profiles; (iii) soil nutrients (N, P, K, organic matter, pH); (iv) rodent incidence; and (v) phylloplane microbial community of the floor vegetation and of apple in the two treatments are planned. Researching the phylloplane microbial community would give an indication of whether kura clover is a potential source of microbes antagonistic to pathogens of apple.
It will be important to confirm that the major source of apple scab for orchards is ascospores and not conidia overwintering in buds.
Treatments Applied to Trees
There is a need to study other, perhaps more sustainable, experimental treatments as alternatives to fungicides. One promising treatment would be compost extracts, which have been shown to have anti-fungal activity, enhanced with nutrients (e.g. spent yeast from breweries) to elevate the population of beneficial microbes on leaves and fruit. Determining the disease control efficacy and potential phytotoxicity on different cultivars will be important for the compounds to gain acceptance by growers.