Resources - Pests

Disease Control

Table of Contents

1. Introduction
2. Rhizoctonia solani
3. Pythium aphanidermatum
4. Phytophthora drechsleri
5. Aphanomyces cochlioides
6. Root Disease Control

1. Introduction

The harvest season is a good time for sugarbeet growers to assess disease losses and to think about the circumstances that may have contributed to the diseases incidence. This is especially true for root diseases, which many times may go unnoticed during the growing season. Harvested beet roots exhibit many disease symptoms. One may notice necrotic lesions on the surface of the beet, hairy roots, rot, or beets which may have vascular necrosis. In this article I will try to describe some of the most noticeable symptoms of sugarbeet root diseases caused by soilborne fungi, and recommend practices which will diminish their severity in future crops.

2. Rhizoctonia solani

Rhizoctonia root rot is caused by the fungus Rhizoctonia solani. This is the most common root disease of sugarbeet, occurring in most beet growing areas in the U.S. as well as throughout the world. Symptoms of rhizoctonia root rot are very apparent at harvest. Crown and root tissue are rotted (see Image #1). Surface lesions are dark brown to black. The rotted tissue often develops fissures under the crowns and will become hollow. These hollow cavities are often filled with the mycelium of the fungus (see Image #2). The rotted tissue due to this disease usually remains dry, unlike the rot that develops with many of the other soil fungi.

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Image #1     Image #2

Growers with a history of rhizoctonia root rot should plant disease resistant varieties. One of the most important disease control practices that a grower can carry out is proper cultivation techniques. The fungus resides in the soil. If cultivation results in soil being thrown in the crowns of the beets, the fungus has a much easier time infecting the plant and causing disease. Root maggot feeding may create a entry point for rhizoctonia infection.  Crop rotation is also an important aspect of disease control. Recent studies have shown that previous crops which contribute a lot of crop residue to the soil, such as wheat or sorghum, will have higher levels of disease compared to cotton, alfalfa, or sunflower as a previous crop.  Some chemical control is being realized with Quadris.

3. Pythium aphanidermatum

Pythium root rot is a serious disease of sugarbeet growing in warm soils. The disease is caused by the fungus Pythium aphanidermatum. The disease occurs in California as well as in the Texas Panhandle. Symptoms of the disease include wilting of the foliage and premature death of the lower leaves. Root symptoms are characterized by black necrosis, proceeding inward through the tap root from lateral roots where the infection had begun (see Image #3). A large portion of the surface of the tap root may be discolored. The disease usually progresses downward through the tap root, and secondary organisms may invade the tissue, causing a soft rot. The diseased tissue is usually sharply delimited from the lower, healthy tissue of the tap root (see Image #4).

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Image #3     Image #4

Because the fungus infects by spores which swim in soil water called zoospores, proper irrigation methods will diminish the incidence of the disease and minimize the amount of spread through the crop. It is important to avoid irrigating for long periods of time, especially when the soil temperatures are high.

4. Phytophthora drechsleri

Phytophthora root rot is caused by the fungus Phytophthora drechsleri. The disease occurs in California when beets are grown during the summer months. The lesions that develop on the surface of the tap root are usually brown (see Image #5). The interior tissue is an amber to reddish brown (see Image #6). Symptoms of the disease usually occur at the tip of the tap root and progress up toward the crown. A sharp delimitation occurs between the diseased lower tissue and the healthy upper tissue of the tap root.

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Image #5     Image #6

Control of Phytophthora root rot, is difficult at best. As with Pythium root rot infection occurs by zoospores and long term exposure to saturated soil during periods of high temperature must be avoided. Fields should have adequate drainage, and beets should be planted on raised beds. If a grower has a history of Phytophthora root rot it would be best to grow the beets under sprinkler irrigation rather than irrigating by furrow.

5. Aphanomyces cochlioides

Aphanomyces root rot is caused by Aphanomyces cochlioides. This fungus also infects the beets by zoospores. Symptoms usually develop first on lateral roots, which turn black on the surface. The infection proceeds to the tap root, causing black lesions on the surface of the tissue. In some cases the basal portion of the root may become fibrous and tasseled; degeneration of the root tissue usually follows.

As with the previous two diseases, good irrigation practices are essential for disease management, especially during prolonged periods of high temperatures. Breeders are working toward incorporating disease resistance into sugarbeet varieties, and as host resistance becomes available, there should be less pressure put on the crop by this disease. A good crop rotation plan will minimize the severity the disease.

6. Root Disease Control

Root diseases of sugar beet are a much different situation for the crop manager compared to foliar diseases. There are few chemical protectants or curatives to use as with leaf spot or mildew. Disease resistance is minimal at best. The disease may go unnoticed for long periods of time. Proper disease management techniques must be thought out ahead of time and incorporated into the general crop production plan for the season. Unlike the foliar diseases which blow into the field, root diseases are endemic in the field soil. Good rotations practices will always help the disease situation. If one expects root disease it is always better to have very high plant populations. Irrigation management is very important. Not only is over watering bad, but a field which becomes too dry will put undue stress on the crop and predispose it to infection. Fields with severe root disease damage will probably be candidates for early harvest.