There have been reports this season of severe outbreaks of high plains disease (HPD) in sweet corn fields in several counties in the Columbia Basin. HPD is caused by the High plains tenuivirus (HPV). The disease was first detected in Colorado, Kansas, and Texas in 1993; in Utah in 1994; and in Washington in 1998.

Hosts: HPV can infect field corn and sweet corn, wheat, barley, yellow and green foxtail, and witchgrass.

Fig. 1. Severe mosaic symptoms on sweet corn caused by the High plains virus.

Fig. 2. Infected (lower) and healthy (upper) plants in a sweet corn crop infected with high plains disease caused by the High plains virus.

Symptoms: Mosaic symptoms are usually first evident on leaves emerging from the whorl, although older leaves may also develop mosaic symptoms (Fig. 1 and 2). Infected plants may be stunted and yellowish (Fig. 3). Yellow or red-purple (depending on the genetic background of the corn cultivar) bands ¼ to ½" in width and running parallel to the veins may be observed on intermediate to fully-developed leaves. These bands later turn necrotic. Infected plants may have stunted or weakened root systems.

Transmission: HPV is transmitted by: 1) the wheat curl mite (Aceria tosichella), a tiny eriophyid mite which also transmits wheat streak mosaic virus (WSMV); and 2) corn seed. Corn plants can be infected with both HPV and WSMV. The mite, which is barely visible with a 10x magnifying lens, can be blown between fields. As a result, a distinct disease gradient is sometimes evident on corn crops planted immediately adjacent to infected wheat crops. Seed transmission is thought to be very low, although the pattern of symptom development in some outbreaks of HPD in sweet corn crops have suggested higher incidences of seed transmission.

Fig. 3. Severely stunted sweet corn plant infected with the High plains virus.

Symptoms

Disease and pathogen

Comments

Peronospora farinosa f. sp. spinaciae on spinach

Note chlorosis on upper leaf surface, and grayish brown felt-like mildew growth on leaf undersurface

Peronospora farinosa f. sp. betae on Swiss chard

Note grayish green felt-like growth and twisted leaf

Peronospora farinosa f. sp. betae on beet

Note pinkish felt-like growth on leaves and twisted seed stalk

Peronospora parasitica on cabbage

Note chlorotic spots on upper leaf surface, and grayish white crust-like growth on leaf undersurface

Peronospora parasitica on broccoli

Note dark streaking in vascular tissue due to systemic infection

Cladosporium leaf spot

Stemphylium leaf spot

Anthracnose

Pathogen name

Cladosporium variabile

Stemphylium botryosum

Colletotrichum dematium

Leaf spot symptoms

Distinct, 1-3 mm spots, develop dark margin

Diffuse, rapidly expanding

Distinct, coalescing, water-soaked in humid conditions

Spores in lesions

+

+

+

Dispersal

Wind, seed

Wind, seed

Splashing water, seed

Overwintering

Volunteers, seed

Woody spinach debris, seed

Volunteers, seed

Favorable conditions

Moist, cool

Moist, warm, pollen on leaves

Wet, cool

Host range

Chenopod species, others?

Spinach

Spinach

Fig. 1. Cladosporium leaf spot (left) and Stemphylium leaf spot (right) on spinach. Photo courtesy of Mike L. Derie.

Fig. 2. Anthracnose on young spinach leaves caused by Colletotrichum dematium. Photo courtesy of Mike L. Derie.

Cercospora leaf spot

Ramularia leaf spot

Phoma leaf spot

Pathogen name

Cercospora beticola

Ramularia betae

Phoma betae

Symptoms

Circular leaf spots, red-brown margin, on older leaves;

Also causes crown lesions

Light brown leaf spots, angular & larger, on older leaves

Round leaf spots, concentric rings on perimeter, dark margin;

Also causes seedling black leg & crown rot

Spores in leaf spots

Minute black dots (stromata)

Silvery gray to white spore masses

Black fruiting bodies (pycnidia), also in crown lesions

Seedborne

+ (external)

+ (?)

+

Dispersal

Splashing water, wind, insects, seed

Wind, seed

Splashing water, insects, seed

Overwintering

Weeds, debris, seed

Debris, seed

Soil, roots, debris, weeds, seed

Favorable conditions

Warm, moist

Cool, moist

Cool to warm, moist

Host range

Beet, chard, Chenopod. weeds

Beet, chard

Beet, lambsquarter

Fig. 1. Cercospora leaf spot on sugar beet.
Photo courtesy of North Dakota State University (https://www.ext.nodak.edu/extpubs/plantsci/
rowcrops/pp1244w.htm)

Fig. 2. Ramularia leaf spot of chard. Photo courtesy of Debra A. Inglis.

Fig. 3. Phoma leaf spot on beets. Photo courtesy of Lindsey J. du Toit.

Alternaria leaf & pod spot

Ring spot

Pathogen name

Alternaria brassicicola &
A. brassicae

Mycosphaerella brassicicola

Symptoms

Black circular to irregular lesions, necrotic center;

Circular to elongated black spots on pods and racemes

Circular lesions (up to 2 cm diameter), definite margin & chlorotic halo, concentric zonation, on older leaves

Spores in leaf spots

Naked spores

Black fruiting bodies (pycnidia &/or perithecia) in concentric rings

Seedborne

+

-

Dispersal

Wind, insects, windblown rain

Splashing water, insects

Overwintering

Debris, biennial seed crops, seed

Debris, biennial seed crops

Favorable conditions

Moist, warm

Moist, cool

Host range

Crucifers

Crucifers

Fig. 1. Alternaria leaf and pod spot caused by A. brassicicola & A. brassicae. Photos courtesy of Lindsey J. du Toit.

Fig. 2. Ring spot of cabbage caused by Mycosphaerella brassicicola. Photo courtesy of Lindsey J. du Toit.

Black rot

Xanthomonas leaf spot

Peppery leaf spot

Pathogen name

Xanthomonas campestris pv. campestris

X. campestris pv. raphani

X. campestris pv. armoraciae

Pseudomonas syringae pv. maculicola

Symptoms

Yellow leaves, wilting, black veins, V-shaped lesions from leaf margin

Circular, water-soaked leaf spots, chlorotic halo; dark lesions on petioles

Circular to angular spots, chlorotic halo

Systemic

+

-

-

Seedborne

+

+

+

+

Dispersal

Splashing water, seed, insects

Overwintering

Debris, cruciferous weeds, soil

Favorable conditions

Warm to hot, moist

Cool to warm, extended wet periods

Cool, wet

Host range

Crucifers (including weeds)

Cabbage, broccoli, cauliflower, kale, radish, tomato, pepper

Cabbage, broccoli, cauliflower, kale, radish, horseradish

Cabbage, broccoli, cauliflower, Brussel sprouts, turnip

Fig. 1. Black rot of cabbage showing blackened veins, V-shaped marginal lesions, chlorosis and wilting. Photo courtesy of Lindsey J. du Toit.

Fig. 2. Xanthomonas leaf spot of crucifers caused by X. campestris pv. raphani & X. campestris pv. armoraciae. Photo courtesy of Lindsey J. du Toit.