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Phytophthora Seedling Damping Off and Root Rot

Causal agent:

The pathogen that causes Phytophthora damping off and root rot of soybean is Phytophthora sojae. Phytophthora exists in soils as populations of different races. Over 70 different races of P. sojae have been detected in Ohio soils, with as many as 50 from a single field. The tremendous variability in the P. sojae populations from these fields indicate that many Rps genes are no longer effective. The final outcome is that P. sojae populations in Ohio have adapted to many of the commercial Rps genes that are currently available in soybean cultivars (Figure 1). Phytophthora sojae populations are in the midst of a race shift in Ohio. This means that not every individual can cause disease on a plant with an Rps gene. Figure 2 illustrates a P. sojae population in one field in Ohio where 100 soil cores were collected, spaced 100 feet apart. P. sojae could be recovered from 82 of the 100 cores indicating a very high population. Next, we analyzed this field for race type or pathotype. The squares indicate those locations where the P. sojae killed plants with the Rps1k gene; circles, Rps1c; and stars, Rps3a. From this field map we can see that any single gene would provide protection in 50% of the locations in the field, but not all.

Symptoms and Signs:

Phytophthora can attack soybean plants at any stage of development. Symptoms in young plants include rapid yellowing and wilting accompanied by a soft rot and collapse of the root. More mature plants generally show reduced vigor and may be gradually killed as the growing season progresses. Foliar symptoms on older plants occur as general yellowing of the lower leaves that progresses upward on the plant, followed by wilting and death.The root system is usually severely affected such that lateral and branch roots are almost completely discolored. Tap roots show a brown discoloration on the surface and, if split, the inner tissues show a tan to brown discoloration. Perhaps the best diagnostic symptom of the disease on susceptible varieties is a lower stem discoloration that may extend several nodes up the stem.

Symptoms on varieties with partial resistance are not as evident as on highly susceptible varieties. When the soil becomes saturated soon after planting, varieties with partial resistance may be subject to damping-off and root rot. However, when infections occur later in the season, the extent of the root damage will be restricted, and there will be no development of the girdling stem lesions as in susceptible varieties.

Disease Cycle:

Phytophthora root and stem rot is caused by Phytophthora sojae. This pathogen survives as thick-walled resting spores, called oospores, which can persist for years in the soil. During periods of adequate soil moisture and temperature, oospores germinate to form structures called sporangia. When the soil becomes saturated, sporangia release small swimming spores called zoospores that are attracted to the soybean roots, to which they attach and germinate. Phytophthora then invades the root and grows within the soybean root cells. Conditions favorable for infection occur most often on heavy clay soils with poor drainage. Phytophthora can attack plants at soil temperatures above 50˚F, but severe disease generally occurs when soil temperatures are 60˚ F or above.

Disease Management:

Host resistance: Choosing the right variety is extremely important when attempting to manage Phytophthora root rot. There are two different types of genetic resistance available in soybean varieties. Race-specific resistance is effective against certain races of the pathogen. The genes for resistance are designated as Rps genes. The second type is partial resistance. Partial resistance is effective against all races of P. sojae, but the level of resistance is not complete, so some level of disease does occur. In problem fields, it is best to choose varieties with race-specific resistance combined with partial resistance to achieve better levels of control. Plant resistant varieties that have race species resistance and/or partial resistance in areas where Phytophthora has been a problem in the past. Varieties are available with resistance genes Rps1, Rps1c, Rps 1k, Rps 3, or Rps 1b+ 3. Only the Rps 1k Rps 1c, Rps 3a or gene combinations of Rps 1k+3a or Rps 1c+ 3a will be effective against the majority of populations that exist in Ohio. There are an increasing number of fields in Ohio in which single Rps genes of Rps 1k or Rps 1c will not be effective due to the adaptation of the pathogen to these Rps genes. In those fields where Phytophthora is a severe problem, combinations of Rps genes with partial resistance are essential.

Soil Drainage: An essential step to control Phytophthora root rot is to improve soil drainage so that flooding is eliminated or minimized. Use cultural practices that reduce soil compaction and improve drainage. Improving drainage is particularly important in no-till soils that retain moisture and require less precipitation to saturate the soil. Phytophthora zoospores are produced only in saturated soil; if soils are not saturated early in the season, varieties with partial resistance will escape disease and remain disease-free throughout the season.

Seed Treatments: In areas where Phytophthora root rot is a consistent problem, fungicide seed treatments can be used to reduce the early season damping-off . To achieve the maximum performance out of soybean varieties with partial resistance, it is important to treat the seeds of these varieties with Allegiance (metalaxyl) or Apron XL (mefanoxam) prior to planting. These fungicides are highly specific for control of Phytophthora damping-off of seedlings. New seed treatments for control of P. sojae include ethaboxam and  oxathiapiprolin, which can be rotated with mefanoxam and metalaxyl to combat pathogen resistance to one fungicide. These seed treatments in combination with genetic resistance provides one of the best options for limiting losses from this disease. To control Phytophthora damping off , especially in poorly drained fields to be planted no-till, the higher rates of these fungicides should be applied.

Fertilizer Applications: Avoid applying high levels of potash, manure, or municipal sludge immediately before planting. The chloride applied with potash, the nitrate applied with municipal sludge, or salt applied with manure will result in more severe root rot. Application should be made in the fall to allow for leaching of soluble salts. Avoid concentrating manure or municipal sludge in specific fields. Spread applications over all fields rather than concentrating on fields close to the source.