Canker formation on apple by Botryosphaeria spp. and control with wound treatments

Introduction

Botryosphaeria spp. are responsible for a variety of symptoms on apple trees, including fruit rots, leaf spots and stem cankers. In warmer climates, fruit rots are the most significant, but in cooler temperate areas, cankers may cause substantial losses. The practice of pruning apple trees during the summer for fire blight management, or to open canopies to light and air may increase the incidence of infection caused by B. obtusa and B. dothidea by wounding the trees at a time when they may be more vulnerable to canker development due to drought stress.

At present, the only control recommended for managing cankers caused by Botryosphaeria spp. is to prune out the affected branches. The availability of effective topical wound treatments to prevent cankers would give apple growers more options for disease management. Two objectives of this study were:

1) To determine whether the virulence of Botryosphaeria isolates on fruit correlated with virulence on stems, with the goal of developing a fruit inoculation assay to screen isolates

2) To test the efficacy of wound treatments in controlling stem cankers caused by B. obtusa and B. dothidea.

Disease Cycle

The fungus overwinters within pycnidia produced on fruit mummies, dead twigs and cankers. Ascocarps are produced on cankered areas of bark and on dead twigs that have been present on the tree over a year. Although conidia are produced throughout the growing season, ascospores are most common within 4-6 weeks of petal fall.  Spores are disseminated by wind, rainsplash, and to some extent, by insects.

The lifecycle of Botryosphaeria dothidea is similar, but leafspot symptoms are absent.

Virulence on apple fruit

Golden Delicious apples were inoculated with 5-day old mycelium (grown on PDA) to determine differences in virulence between isolates of the same species, and between different species. The Wisconsin isolates chosen for subsequent field experiments were similar in virulence to other isolates of the same species tested.

LSD= 0.1705

LSD= 0.2680

* Wisconsin isolates

Means within a trial followed by the same letter are not significantly different.
Values were log transformed for analysis and separation of means, but actual values are shown.

Virulence on Potted Trees

Two-year-old bare root Golden Delicious apple trees were potted in a 2:1 mix of Metromix/ field soil and allowed to grow for 2 months in cold frames before inoculation. Trees were wounded with a 5mm cork borer down to the wood and inoculated with a plug of plain PDA (control) or agar containing mycelium of an isolate of Botryosphaeria dothidea or B. obtusa. The agar was secured in place with masking tape. Wounds were uncovered after two weeks. At eight weeks, the area of the canker (or uncallused wound, if no canker developed) was measured.

Twelve reps of each treatment were used.

Means within an inoculation date followed by the same letter are not significantly different. P=0.05

Virulence on Mature Trees

Mature trees at two locations were wounded by removing a branch 5-9 mm in diameter.   The wound was treated with a field rate of clove oil + ethanol, Thermo-Trilogy neem oil, Garlic-gard, benomyl, kresoxim-methyl, or sterile water (control).  The treatments were applied with a paint brush and allowed to dry. A plug of plain PDA or one containing mycelium of an isolate of Botryosphaeria dothidea or B. obtusa was placed on the wound surface and covered with masking tape. Wounds were evaluated for canker development at 2, 5 and 8 weeks. There were 42 reps of each treatment.

Trees were inoculated on July 16, 1998 and evaluated 8 weeks later.
P= 0.05
Means followed by the same letter are not significantly different.
Values were ranked for analysis and separation of means but actual values are shown.

Trees were inoculated on August 16, 1998 and evaluated 8 weeks later.
P = 0.05
Means followed by the same letter are not significantly different.
Values were ranked for analysis and separation of means but actual values are shown.

Canker Symptoms

Canker symptoms varied from a small halo of dead tissue around the inoculation site to the death of entire branches. Canker development was much more pronounced at Sturgeon Bay, where the trees were experiencing drought stress, than at Arlington, where normal amounts of rainfall occurred. These photographs were taken at Arlington, 8 weeks after the trees were inoculated.

Conclusions

1. There was no correlation between an isolate's ability to rot fruit and its ability to produce stem cankers.
2. There were significant differences among the isolates in their ability to produce stem cankers.
3. In general, on the mature trees, Botryosphaeria dothidea isolates produced larger cankers than isolates of B. obtusa.
4. Differences in canker size among corresponding wound treatments was not strongly pronounced at the Arlington location, where normal rainfall occurred. At the Sturgeon Bay location, under drought conditions, much more extensive cankering developed, and differences among treatments were more apparent.
5. In general, benomyl and the strobilurin-based fungicide, kresoxy-methyl were the most effective wound treatments.

Further testing is being conducted this summer at both locations, and will include a second strobilurin-based fungicide, trifloxystrobin.

Testing of kresoxy-methyl as an airblast-applied spray is also being conducted this summer to determine whether spray coverage using this method of application is adequate to provide control of Botryosphaeria spp.

D. Brown-Rytlewski and P. McManus
University of Wisconsin, Madison 53706
August 1999