Diseases and Pests
Diseases and pests are generally attracted to sugarbeets. The following is a list of the most common ones and common symptoms associated with them:
The sugarbeet nematode, Heterodera schachtii, is a major parasite of sugarbeets. It causes serious stand and yield reductions wherever sugarbeets are grown. Entire fields may be infested, or they may have one or more localized areas of infestation. Localized infestations may result in well-defined circular or oval areas where plant stands and growth are poor. Over time these areas usually become enlarged and spread out. H. schachtii can parasitize roots of plants of all ages. When infected, outer leaves of plants usually wilt during the hot period of the day or when soil moisture becomes limited. Leaves of parasitized plants also may have pronounced yellowing. Affected plants have small storage roots that are severely branched with excess fibrous roots often referred to as "bearded" or "whiskered". When older plants are attacked, symptoms are less noticeable.
Cercospora Leaf Spot
Cercospora leafspot, caused by the fungus Cercospora beticola, is one of the most serious diseases of sugarbeets. This disease can cause reduced tonnage and increased impurities. Roots of affected plants do not store as well in the pile as roots of healthy plants. Cercospora infection of the sugarbeet leaf produces circular spots about 1/8 inch in diameter with ash gray centers and dark brown to reddish purple brown borders. During warm, rainy, humid weather, the spots may coalesce and kill entire leaves, particularly on susceptible varieties. In humid weather, these coalescing spots may be covered with areas of steel blue to light bluish-purple fuzz. These are masses of spores of the Cercospora fungus. Severely diseased leaves wither and die, resulting in severe defoliation. The disease begins on the older leaves and progresses to the younger leaves. Diseased leaves usually remain attached to the crown of the plant.
Rhizomania, "crazy root" or "root madness", is a serious disease that affects sugarbeets. Rhizomania can greatly reduce sugar yield by reducing either the tonnage or sugar content, or both. Classical root symptoms following early infection include a mass of fine, hairy secondary roots, mostly dead, that give the taproot a beard-like appearance. With slightly later infection, the storage root often is rotted and constricted, becoming much broader near the crown, thus resembling the shape of a wine glass. Longitudinal sections of infected roots reveal vascular tissue that is visibly darkened. Infected roots occasionally rot. Very late infections may result in no obvious symptoms.
Fusarium oxysporum f. sp. betae survives in soil and plant residues as spores, chlamydospores, or mycelium. When conditions are favorable, the fungus enters sugarbeet roots and invades the vascular system where it produces toxins that are transported upward in the plant, causing foliar symptoms. The disease is favored by high soil temperatures -- above 75°F; symptoms typically do not appear early in the growing season. Fields that are waterlogged or with poor soil structure provide favorable conditions for infection. In early stages of the disease, foliage tends to wilt during the day but recovers overnight. Entire leaves eventually die but remain attached to the plant and collapses in a heap around the crown. There are no external root symptoms. A transverse section through the root shows a grayish brown vascular discoloration. Mature plants rarely die, but yield is reduced.
Rhizoctonia Root and Crown Rot
Rhizoctonia destroys feeder roots and ultimately invades the taproot, producing dark, oval lesions. The lesions are initially shallow and scab-like in appearance, but may enlarge and girdle the taproot. Because the taproot is not rotted completely in the early stages of the disease, plants may remain alive if affected only by the root rot phase. Regrowth is slowed due to the reduced ability of the root system of affected plants to absorb water and nutrients. During cool months of the year when the fungus is inactive, lesions may heal over and new feeder roots may be regenerated. Although Rhizoctonia is capable of causing crown rot in the absence of other organisms, it is frequently found in combination with Fusarium, Phoma, and Colletotrichum.
Beet Curly Top
Beet Curly Top Virus (BCTV) is widespread throughout the western United States, and to a limited extent on the eastern slope of the Rocky Mountains. It is transmitted only by the leafhopper, Circulifer tenellus. Leaves are dwarfed, crinkled, and rolled upward and inward. Veins are roughened on the lower side of the leaves and often produce swellings and spine-like outgrowths. Roots are dwarfed, and rootlets tend to become twisted and distorted. Proliferation of rootlets will occur later; this is also known as hairy root. Phloem tissue often becomes necrotic, cracks develop, and phloem exudates appear on stems and leaves. Dark areas of necrotic tissue can usually be observed in root sections of diseased plants. Necrotic areas appear as dark rings in transverse sections and as dark streaks in longitudinal sections. The severity of an attack of curly top depends on climate factors affecting the weed hosts of the virus, the prevalence and severity of the virus, and the reproductive capacity and migration of the leafhopper.
Aphanomyces is sometimes referred to as black root. Pythium ultimum trow is present to some extent in nearly all arable soils and attacks unprotected seedlings at all temperatures favorable for the germination of beet seed. The fungus is favored by high soil moisture and attacks seedlings of many other crops. Infection primarily causes preemergence damping-off. Postemergence damping-off may follow under moist conditions.
Pythium aphanidermatum, a high-temperature fungus, attacks seedlings only in warm soils with abundant soil moisture.
Phoma betae Frank causes preemergence damping-off only if heavily infested seed is planted at a low temperature in soils with abundant moisture. Severe infection of the hypocotyl may appear, resulting in retarded growth and dark brown discoloration.
Most of the seedling pathogens are soilborne. The only important seedborne pathogen of sugarbeet seedlings is Phoma betae.