Berry Disease Snapshot: Mycosphaerella Leaf Spot of Strawberry

By David Strickland and Kerik Cox

Cornell AgriTech

Disease Snapshot: Mycosphaerella Leaf Spot of Strawberry (Bird’s Eye Spot, White Spot)

Causal Pathogen: Mycosphaerella fragariae

When to watch for it: Year round

First line of defense: Plant disease-free nursery stock

Summary:

Infections can occur on fruit (black seed), petioles, stolons, and leaves. Foliar lesions first appear as irregular purple spots on both the upper and lower surfaces of the leaf. Lesions develop a brown to white center as they mature, with a rusty brown border surrounding the necrotic area of the lesion. Severe infection on a susceptible cultivar will kill the leaf as lesions coalesce over the entire surface. Symptom expression varies by cultivar, temperature, and strain of M. fragariae. M. fragariae can overwinter as lesions on living leaves under snow cover, as conidia from overwintering dense masses of mycelia (sclerotia), and as sexually recombinant ascospores. Conidia from sclerotia are rain-dispersed from dead leaves in the field in the early spring, and ascospores are disseminated by wind. Virtually all parts of the plant are susceptible regardless of age, but lesions are most severe on middle-aged leaves. Severely infected plants are less likely to survive the winter, and those that do will have a reduced number of flowers in the spring.

Resistance reports have varied by state, and resistant cultivars may or may not be an effective means of disease control. Removing dead plant matter before winter will reduce inoculum carry over between seasons.  Fungicide applications are recommended for plantings with susceptible varieties in the early season to eliminate overwintering inoculum.

Berry Disease Snapshot: Rhizopus Rot of Strawberry (Leak)

Soft, collapsed strawberries that begin to mold can be a sign of rhizopus rot.

By David Strickland and Kerik Cox
Cornell AgriTech

Berry Disease Snapshot: Rhizopus Rot of Strawberry (Leak)

Causal pathogen: Rhizopus stolonifer, R. sexualis

When to watch for it: During the fruit-ripening period

First line of defense: At harvest, rapidly cool harvested fruit below 45°F

Summary:

Primarily a postharvest rot, Rhizopus rot may also occur in the field on ripe fruit. Modern storage and shipping have reduced this disease’s incidence worldwide, but it may readily appear if fruit are not cared for properly. Fruits infected with Rhizopus spp. appear slightly discolored, gradually turning light brown. The fruits rapidly soften and collapse, allowing their juices to leak out. Under humid conditions, white fluffy mycelium with large black sporangia appear. Rhizopus spp. can successfully overwinter on leftover plant material and in soil.

Protective broad-spectrum fungicides can be used to control Rhizopus rot in the field. Rhizopus spp. can only infect wounded fruit, so a vigilant insecticide program to manage fruit-feeding insects is encouraged. At harvest, Rhizopus rot is best controlled by rapidly cooling berries and maintaining storage temperatures below 45°F.

Berry Disease Snapshot: Black Root Rot of Strawberry

By David Strickland, Kerik Cox
Cornell AgriTech, Geneva, NY

Black Root Rot of Strawberry

Causal agent: many soil-borne fungal species, abiotic factors

When to watch for it: During the growing season

First line of defense: Good soil health maintenance practices

Summary:
A disease complex, black root rot does not have one specific cause. It may be caused by injurious environmental conditions such as freezing or waterlogged soils, root-lesion nematodes, and/or various soil-borne fungal pathogens alone or in combination. The disease has been observed on plants grown in soils with a high clay content. The most prevalent symptoms of black root rot are death of feeder rootlets, deterioration of structural roots, and declining plant vigor.

Maintenance of soil health is the best control against black root rot. Soil compaction reduces soil aeration, promoting conditions conducive to disease development. Poor irrigation practices and waterlogged soil does the same. Chemical fumigation of the soil is not recommended as the causal agent may not be fungal.

Berry Disease Snapshot: Angular Leaf Spot of Strawberry

By David Strickland and Kerik Cox

Cornell AgriTech, Geneva, NY

Disease: Angular Leaf Spot of Strawberry

Causal pathogen: Xanthomonas fragariae

When to watch for it: Year round

First line of defense: Purchase disease-free stock

Summary:

Angular leaf spot of strawberry has been introduced to major production areas by the importing of infected plant material. The bacterial pathogen, X. fragariae, is very resistant to desiccation and can survive adverse conditions for extended periods in or on dead plant matter both in transit and in the field. The pathogen cannot overwinter freely in soil, which implies that one key cultural control is the removal of old strawberry plant matter before new fields are planted.

Young, vigorous plants are more likely to be infected during the spring when temperatures remain around 68°F during the day and when the leaf wetness is prolonged by rain or sprinkler irrigation. Symptoms begin as water-soaked lesions that enlarge but are delimited by veins, creating characteristic angular spots. During moist periods, viscous bacterial ooze may be visible on the undersides of infected leaves, which dries to a white film. Severe infections will result in the death of the leaf.

Preventing the introduction of X. fragariae on transplants has proven to be more effective than chemical control measures. If absolutely necessary, applications of streptomycin sulfate and oxytetracycline have been shown to be effective protectants in the early season. Applications of copper ammonium carbonate and cupric hydroxide have also been effective protectants in 6 day intervals. However, greater than 7 applications risks the development of severe phytotoxic symptoms. For organic production, the use of hydrogen peroxide at its highest labeled rate is suggested if disease pressure is severe.

A: Bacterial lesions observed on the underside of an infected strawberry leaf. In direct (transmitting) light, the lesions are transparent. Unlike in B: where the lesions are a different shade of green in reflected light, which is a characteristic feature for identification. C: the underside of a severely infected strawberry leaf.

Managing Strawberry Fruit Rots with Biopesticides

A special thank-you to Dr. Kerik Cox for sharing this article in our May newsletter. Because we just didn’t have enough space to run all of his informative graphs, we’re including the remainder here. Refer to your print newsletter to read the text of the article in its entirety.

Figure 3. Mean percent incidence of Botrytis fruit rot at harvest in October 2018 in a day-neutral planting of ‘Albion’ strawberries with application timings using ‘NEWA’ or on a ‘Calendar’ schedule for organic (green) and conventional (orange) fungicide programs. Values represent means and standard errors of six plots. Columns denoted by the same letter for plots under “cover” (low tunnels) or “uncovered” are not significantly different (P < 0.05) according to the LSMEANS procedure in SAS 9.4 with an adjustment for Tukey’s HSD to control for family-wise error.

Figure 4. Mean percent incidence of Botrytis fruit rot at harvest in October 2018 in a day-neutral planting of ‘Albion’ strawberries planted on open plastic (cover) or under low tunnels (no cover) for calendar timings (yellow 7-10 days) and NEWA strawberry fruit rot systems (blue “High” risk). Values represent means and standard errors of six plots. Columns denoted by the same letter capital (plots under tunnels) or lowercase (uncovered plots) are not significantly different (P < 0.05) according to the LSMEANS procedure in SAS 9.4 with an adjustment for Tukey’s HSD to control for family-wise error.