Andy Wyenandt

This is an archive of Dr. Wyenandt's posts on the Plant and Pest Advisory.

April 16, 2020

Identifying and controlling common leaf spot in strawberry

Strawberry leaf spot, caused by the soil-borne fungal pathogen, Mycosphaerella fragariae, has been found in some strawberry fields on some cultivars this past week in southern New Jersey. Leaf spot can infect leaves, petioles, runners, fruit stalks (pedicels), and berry caps or calyxes. Small, dark purple to reddish-purple, round spots, 1/8 to 1/4 inch in diameter (3 to 6 millimeters), appear on the upper leaf surfaces. The center of the spots soon become tan or gray and eventually almost white, while the broad margins remain dark purple. Later in the season, dark specks (sclerotia and/or perithecia) may be seen in the older lesions. Tannish areas form on the underleaf surface. The symptoms on the other plant parts, except the fruit, are almost identical to those that develop on the upper leaf surface.

Strawberry leaf spot on infected leaves

Strawberry leaf spot on infected leaves.

Symptoms of strawberry leaf spot on infected leaf petioles

Symptoms of strawberry leaf spot on infected leaf petioles.

Microsclerotia and conidia from infected leaves that survive the winter can lead to infections of new growth in the spring. The period between infection and the appearance of lesions on the upper leaf surface can range from 10 to 14 days depending on weather conditions. Large numbers of conidia can cause secondary infections during prolonged periods of damp to wet, moderately warm weather. Temperatures between 65 and 75 F (18 to 24 C) are optimal for the growth of fungus and for lesion development. Infections can continue to occur throughout the growing season. Young, expanding leaves are much more susceptible to infection than mature leaves. If frequent rains occur during early- and mid-spring, a few infection sites can start an epidemic.

 

Control of strawberry leaf spot begins with recognizing symptoms and preventative fungicide applications. All strawberry fields need to be scouted on a regular basis. Especially, during periods of wet weather or during heavy use of overhead irrigation. Weekly applications rotating the following should be done as long as symptoms are present and weather conducive for disease development persists. For more information please see the 2020/2021 Mid-Atlantic Commercial Vegetable Recommendations Guide. Fungicide applications for strawberry leaf spot will be effective for strawberry leaf scrorch and strawberry leaf blight.

Code Product Name Product Rate Active Ingredient(s)

(*=Restricted Use)

 PHI

(d)

 REI

(h)

 Bee

TR
M04 Captan 50W 6.0 lb/A captan 0 24 N
M04 Captan 80WDG 3.7 lb/A captan 0 24 N
M04 Captan 4L 3.0 qt/A captan 0 24 N
M04+17 Captevate 68WDG1 3.5 to 5.25 lb/A captan + fenhexamid 0 24 N
Do not apply the same FRAC code more than twice in a row or in a season.
1 Topsin M 1.0 lb/A thiophanate-methyl 1 24 N
2 Meteor2 1.5 to 2.0 pt/A iprodione n/a 24 N
2 Nevado 4F2 1.5 to 2.0 pt/A iprodione n/a 24 N
2 Rovral 4F2 1.5 to 2.0 pt/A iprodione n/a 24 N
3 Rally 40WSP 2.5 to 5.0 oz/A myclobutanil 0 24 N
11 Cabrio 20EG 12 to 14 oz/A pyraclostrobin 0 12 N
3 + 11 Quadris Top 1.67SC 12 to 14 fl oz/A difenoconazole + azoxystrobin 0 12 —
3 + 11 Quilt Xcel 2.2SE 14 fl oz/A propiconazole + azoxystrobin 0 12 N
7 + 11 Merivon 2.09SC 4 to 7 fl oz/A fluxapyroxad + pyraclostrobin 0 12 N
7 + 11 Pristine 38WG 18.5 to 23.0 oz/A boscalid + pyraclostrobin 0 12 —

 

For organic strawberry growers, weekly applications of an OMRI-approved copper or potassium bicarbonate have been shown to be effective in mitigating strawberry leaf spot. Please see link to MSU’s fungicide efficacy table for organic and conventional fungicide use.

For information on controlling angular leaf spot in strawberry please click here.

For information on controlling fruit rots in strawberry please click here.

For more information on strawberry leaf spot please see the following:

University of Illinois – http://ipm.illinois.edu/diseases/series700/rpd702/index.html

University of Wisconsin – https://hort.extension.wisc.edu/articles/common-leaf-spot-of-strawberry/

Michigan State University, 2015 – Information on diseases and fungicide efficacy – https://www.canr.msu.edu/news/protect_strawberries_from_foliar_diseases_after_renovation

Cornell University, 2013 – Information on diseases and fungicide efficacy – https://cpb-us-e1.wpmucdn.com/blogs.cornell.edu/dist/0/7265/files/2017/01/strleafdisidmgmt-yjcu5n.pdf

Please remember, the label is the law!

Filed Under: Vegetable Crops Tagged With: , ,
April 12, 2020

Edema development in brassica crops

Edema is often expressed as off-color swellings or galls that appear on leaves and stems. Edema develops when epidermal cells hold excessive water due to a slowing of evapotransporation when hot, muggy days are followed by cooler, wetter weather. Edema develops because the plant takes in more water (due to a high soil moisture contant) faster than it can get rid of it through evapotranspiration causing cells to rupture which results in the blistering of the leaves. Edema is strictly caused by environmental factors and can appear whenever these conditions are met. Properly monitor soil conditions, irrigation cycles, and the weather to avoid over irrigating on warm, hot early spring days, especially when quick cold fronts/temperature drops and cloudy weather are expected.

Symptoms of edema on collard leaf. Note the off-color appearance of leaf surface.

Symptoms of edema on top side of
collard leaf.
Note the off-color appearance of leaf surface.

Edema, bottom side of collard leaf. Note the irregular, 'corky appearance caused bythe rupture of leaf cells.

Edema, bottom side of collard leaf.
Note irregular, “corky” appearance due to leaf cell rupture.

Filed Under: Vegetable Crops Tagged With:
April 10, 2020

Understanding and controlling angular leaf spot in strawberries

Often considered a minor pathogen, angular leaf spot caused by the bacterium, Xanthomonas fragariae, can cause serious leaf and calyx infections ruining the marketability of fruit if left uncontrolled. Like all bacterium, the pathogen will infect leaves and the calyx through natural openings or wounds. Primary infections of new growth in the spring originate from systemically infected overwintered plants and dead leaves in which the bacterium survives the winter; or from infested transplants. The pathogen is very resistant to desiccation and can survive in old, dried leaves or infected plant debris buried in the soil. The pathogen will not survive free in the soil so it originates primarily from infected leaf debris and infected crowns.

Fig. 1. Water-soaked lesions caused by angular leaf spot on infect strawberry leaf. Photo by P. Nitzsche

Infections can often start in production operations and come in on infected bare root transplants or cuttings. Symptoms on leaves include initial small, irregular water-soaked lesions (Fig. 1). Young, actively growing leaves are most susceptible, especially on vigorously growing plants. Disease development is favored by moderate to low daytime temperatures, low night time temperatures – near or below freezing, and high relative humidity. Long periods of precipitation, overhead irrigation used to establish plantings or protect plantings from freezing, and heavy dews favor disease development.
[Read more…]

Filed Under: Vegetable Crops Tagged With: ,
April 9, 2020

Identifying and controlling botrytis in high tunnel and greenhouse tomato production

Botrytis, or gray mold, caused by the fungus, Botrytis cinerea, can cause significant losses in high tunnel and greenhouse tomato production if not controlled properly. The pathogen can rapidly spread during periods when structures are closed and when relative humidity remains high for long periods of time. This often occurs when outside weather remains cool and damp while heating is needed. Gray mold is favored by temperatures from 64° to 75°F and requires only high humidity (not leaf wetness) to become established. The pathogen has a large host range and once established in an enclosed structure it can be very difficult to control (UMASS). The fungus can survive/overwinter as mycelia or sclerotia in plant debris and in organic soil matter (NCSU).

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Filed Under: Vegetable Crops Tagged With: , , ,
April 9, 2020

Identifying and controlling leaf mold in high tunnel & greenhouse tomato production

Leaf mold occasionally appears in high tunnel or greenhouse tomato production in New Jersey. However, under ideal conditions the disease will develop in field-grown crops. The fungus will cause infection under prolonged periods leaf wetness and when relative humidity remains above 85%. If relative humidity is below 85% the disease will not occur. Therefore, the proper venting of high tunnels and greenhouses on a regular basis is important. The pathogen can survive (overwinter) as a saprophyte on crop debris or as sclerotia in the soil. Conidia (spores) of the fungus can also survive up to one year in the soil.

Symptoms of leaf mold on infected tomato plant. Note the bright yellow leaves and the olive-green spores developing on the undersides of leaves.

Symptoms of leaf mold on infected tomato plant. Note bright yellow leaves and olive-green spores developing on undersides of leaves.

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Filed Under: Vegetable Crops Tagged With: , , ,
April 9, 2020

Identifying white mold in greenhouse & high tunnel tomato production

White mold, or timber rot, is caused by the soil-borne fungal pathogen, Sclerotinia sclerotiorum.

High tunnel tomato plant with main stem infected by white mold

High tunnel tomato plant with main stem infected by white mold.

The photo on right shows a high tunnel tomato plant with main stem infected by white mold. Over time the entire plant will slowly collapse looking similar to a wilt or root rot pathogen as the water supply is cut off to the plant.

White mold is common and once introduced into a field or high tunnel it can very difficult to control. The pathogen produces black sclerotia on the surface and inside infected stems. Sclerotia, if they make their way back into the soil, can survive for years causing significant problems.

Infected stems will turn a light brown color and dry up becoming brittle. If the main stem is infected the entire plant will collapse looking like a wilt or root rot issue.

Under hot, humid conditions white fungal growth will develop on the surface of the stems and in some cases sclerotia will develop on the outside of the stem. Breaking open the stem will reveal numerous black sclerotia.

All infected plants and or plant material need to be removed from the greenhouse or high tunnel immediately and disposed of properly. Preventative protectant fungicide programs beginning at flowering will help control white mold. Cool, wet weather and poor air circulation favors disease development.

There are a number of control options for conventional and organic growers listed in Table E-11 in the 2020/2021 mid-Atlantic Commercial Vegetable Production Recommendations Guide.

Tomato stem infected by white hold. Note the fungal growth on the surface of the stem.

Tomato stem infected by white hold. Note the fungal growth on the surface of the stem.

White mold of tomato. Note the black sclerotia developing inside the brittle stems.

White mold of tomato. Note the black sclerotia developing inside the brittle stems.

Filed Under: Vegetable Crops Tagged With: , , ,