Resources - Pests |
Disease Control |
Table of Contents |
1. IntroductionThroughout all of Holly Sugar's production areas, a frequent topic of conversation between Holly's agriculturist and growers is the importance of obtaining and maintaining adequate plant populations. High populations are needed to obtain the greatest yield and quality, especially under such conditions as short growing seasons or growth limiting disease pressures. A primary requirement of achieving the goal of high plant populations is control of seedling diseases.
2. Seedling Disease InfectionSugarbeet seedling diseases are caused by a variety of soilborne and seedborne fungi. These fungi infect the succulent young seedling shortly after germination. The infected seedlings usually decay at the soil level. They will fall over because of a lack of supporting tissue during this early stage of growth (see Image #1). This process is known as damping off. The name is derived from of the moist decay of the seedling and the moist conditions under which the diseases occur. Click image for full page view.
3. Damping OffTwo distinct forms of damping-off are recognized. Pre-emergence damping-off occurs before the seedlings ever arise from the soil. In this phase no direct symptoms of the disease are visible above the soil surface. Post-emergence damping-off occurs after the seedlings have appeared above the soil surface (see Image #2). This article describes the most common seedling diseases of sugarbeet and outlines control measures that will minimize the damage caused by these maladies. Click image for full page view.
4. Aphanomyces cochlioidesAphanomyces seedling disease is caused by the same pathogen that causes black root rot, Aphanomyces cochlioides. This soilborne pathogen causes post-emergence damping-off of sugarbeets in the seedling stage. The fungus survives in the soil and infects the host by swimming spores called zoospores. The zoospores must have free water to swim to the host tissue and infect. Symptoms of Aphanomyces seedling disease are unique. Infection occurs on the root or hypocotyl at or just below the soil level (see Image #3). As the infection spreads up the hypocotyl, it begins to darken and will turn jet black. The cortex of the hypocotyl dries and shrinks to a dark slender thread and the seedlings will weaken and fall over. Characteristically, the cotyledons will remain green and turgid during the initial phase of the disease, but eventually the seedlings die. Click image for full page view.
Aphanomyces seedling disease is most prevalent when soil temperatures are greater than 70° F., and may be very severe. In conditions of warm, moist soil, seedlings stands can be devastated. Replanting of fields after stand loss is not likely to be effective due to an increase in inoculum concentration, and warmer soil temperatures. Control of this disease is through an integration of cultural practices. Fields to be planted should have conditions of friable, well drained soils, and be free of weeds such as lambsquaters and pigweed, which can harbor the pathogen. The fields should be planted before soil temperatures warm above 60° F. High soil phosphate promotes vigorous growth that will reduce the time seedlings are susceptible to infection. Rotations with non-susceptible crops will reduce the inoculum in the soil and the probability of severe disease occurrence. No chemical seed treatments are available in the United States for the control of this disease.
5. Rhizoctonia solaniRhizoctonia solani is a soilborne fungus that will cause both pre-emergence as well as post-emergence damping-off of sugarbeet seedlings. In contrast to Rhizoctonia root rot, caused by a single strain of the pathogen, several different strains of this fungus will cause seedling disease on sugarbeets. The pre-emergence phase of the disease occurs when the fungus infects the seedling as it is growing from the seed. The seedling is killed and never emerges from the soil. Symptoms of the post-emergence phase of the disease can be very distinct, they include stunting and chlorosis (see Image #4). Dark brown and black lesions appear on the root and hypocotyl. The lesions may spread the entire length of the hypocotyl, but it does not shrink and become thread-like as with Aphanomyces seedling disease. Click image for full page view.
Rhizoctonia seedling disease usually is most severe with warm soil temperatures, 70-85° F. Moist soil conditions will contribute to severity of this disease; but, the pathogen is not as dependent on soil moisture as Aphanomyces. Crop rotation can dramatically affect the severity of this disease. Previous crops that contribute a lot of residue to the soil, such as grains or alfalfa, make the disease more severe in a following sugarbeet crop. Control of this disease is by fungicide seed treatment. Chloroneb and Thiram are both very effective. Planting seed shallow will control the disease by minimizing the area of the hypocotyl exposed to the pathogen.
6. Pythium spp.Pythium spp. are very common seedling disease pathogens. These soilborne pathogens infect a wide variety of crops, in many different soils and areas. On sugarbeets, these pathogens cause both pre-emergence as well as post-emergence damping-off. The pre-emergence phase is more common. These pathogens can infect the seed very quickly after planting, inhibiting their germination and resulting in poor stands. Post-emergence symptoms include wilting and total tissue collapse (see Image #5). The root, hypocotyl, and cotyledons become water soaked, turn dark in color, and collapse very quickly. These symptoms are followed immediately by seedling death. Click image for full page view.
Two species of Pythium are responsible for these diseases. In wet, cool soils (< 65° F.) Pythium ultimum frequently occurs. This is the most common of the two species, occurring in most of the sugarbeet growing area of the United States. In the warmer parts of the country, such as California and Texas, where sugarbeets are planted into warm soils (> 80° F.), Pythium aphanidermatum can be an important seedling pathogen. Both pathogens are more severe under wet conditions. These pathogens have a wide host range and are able to saprophytically colonize crop residue, so crop rotations are not effective for control. Good irrigation practices, such as pre-irrigating before planting, may help reduce the amount of infection and stand loss. Seed treatment fungicides are very effective for control. Both Captan and Apron are used throughout the industry to control these pathogens.
7. Phoma betaePhoma betae is the only seedborne pathogen that is capable of causing seedling disease. Seed infection occurs in the seed production fields where during cool wet periods the fungus will infect developing floral parts. The fungus colonizes the developing seed. It can cause pre-emergence damping-off when the seed is planted. The disease usually only occurs under very wet and cool soil conditions (< 50° F.), and only then if the seed is infested with the pathogen. Control of this pathogen is provided mainly by seed production techniques. Cultivation and harvesting of sugarbeet seed include methods that minimize the exposure of the seed to periods of cool and wet conditions. Processing of the seed removes the outer layers of the cortical tissues that are most likely to harbor Phoma betae. Thiram seed treatment is also affective for control of this pathogen.
8. Seedling Disease ControlSeedling disease control is a very important aspect of sugarbeet production. Loss of stand due to disease is time consuming. Re-planting sugarbeet fields is expensive, due to direct cost such as seed and operational expenses, as well as to indirect cost. The most significant indirect cost is reduced yield due to a shorter production season. Also, it is difficult to reestablish sugarbeet stands after loss due to seedling disease. Fields to be replanted usually have increased inoculum concentrations, and increased pathogen activity due to higher soil temperature. Adequate control of seedling disease will help assure the necessary plant population needed for a healthy sugarbeet crop. Click image for full page view.
The use of seed treatments is one way to help establish a healthy plant population. When done properly, seed treatment protects the seed from fungi associated with the seed. Spores of these fungi adhere to the seed. When the seed is planted, these fungi begin to grow, invade the seed or seedling, and cause a seedling blight. Other fungi live in the soil and may cause seedling problems. Soil-borne Pythium, Aphanomyces and Rhizoctonia fungi can cause serious stand loss when the soil is moist or wet. Tachigaren seed pelleting is highly effective against Pythium at lower rates and Aphanomyces at higher rates. It persists for only 3-4 weeks and will provide protection only for the emerging seedling; it does not provide protection against mid-season infection. For season-long management of Aphanomyces, the best approach is to apply Tachigaren to varieties with partial resistance to Aphanomyces. Early planting and cultivation to help dry out the soil may also help reduce early season losses from Aphanomyces seedling disease. Tachigaren is not effective against Rhizoctonia, so an Aphanomyces soil test is important to determine what management practices to use.
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