Common Home Garden Asparagus, Rhubarb, and Horseradish Diseases

Date: January 20, 2024

Whether you're planning your garden for later this year or already getting plants in the ground...it's never too early to learn about the plant disease symptoms you need to know. Early detection of plant diseases is crucial to protect your crop.

What is a pathogen?

A pathogen is an organism causing disease to its host. Bacteria, viruses, nematodes, and fungi are pathogens that can cause plant diseases. Pathogens enter their host and seize the plant's energy for their use. Pathogens can enter a plant in various ways, including wounds or natural openings, penetrating plant tissue, mechanical damage, or insect transmission.

What is a disease cycle?

A disease cycle is a series of events involved in disease development. In most disease cycles, the basic steps are inoculum production, the spread of inoculum to a susceptible host, penetration of inoculum into a host, infection, secondary cycles, and pathogen survival between host plants.

Why is identification vital?

Identifying pathogens is the first step in controlling plant diseases and producing quality crops. Proper identification is necessary to implement effective management strategies.

ASPARAGUS

Purple Spot

The fungus Stemphylium vesicarium causes purple spot disease.

D. Johnson, Washington State University

Symptoms

Symptoms include sunken, reddish-purple lesions and may develop a brown center if the lesion is large. Lesions can be found on the lower half of the spears and do not impact the internal tissue of the spear. This disease may cause blighting of foliage, which may significantly reduce yield the following year.

Disease Cycle

Thrives in cool, wet spring weather. The fungus overwinters on crops and fern debris. In spring and summer, fruiting bodies on plant debris produce spores. Spores may be wind-carried.

Cercospora Blight

The fungus Cercospora asparagi causes the foliar disease Cercospora blight in asparagus.

John Damicone, Lynn Brandenberger, Oklahoma State University Extension

Symptoms

Symptoms include browning of the ferns and small, greyish-tan spots with reddish-brown borders on the needles and small branches. These lesions can first be found on the lower part of the fern and progress upward. In severe cases, ferns will yellow over time. This disease results in early defoliation and loss of crown vigor, causing a reduction in the yield of spears the next spring.

Disease Cycle

Thrives when daily temperatures average 77–86°F and with relative humidity 95% or above. The fungus overwinters on fern debris in the soil. Rain and wind disperse spores.

Asparagus Decline Syndrome

Asparagus decline syndrome has been linked to the presence of the fungus Fusarium in the soil.

John Damicone, Lynn Brandenberger, Oklahoma State University Extension

Symptoms

When plants are affected by asparagus decline syndrome, productivity will slowly lessen. The yield will be lower, the spear size will reduce, and the crown will eventually die. Symptoms include root rot that leads to a significant decrease in feeder roots. The crown will appear discolored, and adult plant fronds will wilt and die.

Disease Cycle

Many factors can contribute to the decline of asparagus plants. These factors include winter crown injury, weeds, insects, acidic soil, several diseases, and soil compaction.

RHUBARB

Gray Mold

The fungus Botrytis cinerea is responsible for gray mold on rhubarb plants.

Dan Egel, Wenjing Guan, Purdue University *This photo is Botrytis cinerea on a tomato plant.

Symptoms

The first symptom of this disease is grayish-brown discoloration on the plant’s leaves. Under humid conditions, the affected areas will develop into gray, fuzzy growth. The stalks may turn red-brown and develop water-soaked areas on stems.

Disease Cycle

This disease overwinters on decayed plant parts. Spores may also be abundant in the air.

Phytophthora Root and Crown Rot

The soilborne fungus Phytophthora cactorum causes phytophthora root and crown rot.

Amgrow Specialty Agriculture

Symptom

Infected plants will experience leaf wilt, fail to leaf out in the spring, and experience premature leaf death. Depressed, brown lesions will be visible on the base of the stalk, causing the entire stalk to collapse. When examining crowns, the roots will be brown-black, damaged, and disintegrate.

Disease Cycle

This disease thrives in cool, rainy weather and favors heavier, clay-like soils.

HORSERADISH

Bacterial Leaf Spot

The bacteria Phytomonas campestre armoraciae causes bacterial leaf spot in horseradish plants.

Photos courtesy of Margaret McGrath, Cornell University

Symptoms

Symptoms will first appear as small, dark green, translucent spots. These spots eventually turn brown/black and scatter across the leaf's surface. Leaves may curl up, become dry, and drop. It is imperative that leaves do not stay wet for long periods.