- Author: Richard Smith, Vegetable Crop and Weed Science Farm Advisor
In recent weeks a number of samples have come into our office of lettuce plants that have the following symptoms: stunting, yellowing outer leaves and occasionally with wilting during the afternoon (Photo 1). The symptoms superficially resemble Lettuce Dieback caused by Tomato Bushy Stunt Virus, but Steve Koike has not detected this virus in these plants. Affected plants also typically have roots that are no longer than 1.5 to 2.0 inches long (Photo 2). Upon careful examination of the root tissue, it can be seen that the roots once extended further, but were burned off at this point in the soil. The death of the tip of the root was not caused by a disease. In nearly all cases that I have seen so far, this problem occurs on heavier clay loam to clay type soils.
Based on the uniformity of the depth of the point of death of the tap root, it appeared that this problem was associated with an application of fertilizer. Given that fertilizer is a salt, it is capable of damaging young root tissue. These symptoms are distinct from ammonium toxicity which damages lettuce root tissue by the toxic action of the ammonium on root tissue (see Blog entry April 26 by Steve Koike). Ammonium toxicity causes distinct symptoms on affected roots; however, these symptoms are distinct and appear to be caused simply by salt burn of fertilizer. (Photo 3).
To confirm this hypothesis, last summer I worked with a cooperating grower to recreate these symptoms on lettuce. I used a pipette to inject fertilizer 0.5, 1.0 and 2.0 inches from the base of lettuce plants, and 1.5 inches deep in the soil; all applications were applied at the thinning stage. We observed that there were higher levels of plants with the tap root burned off in the plots where the fertilizer was applied 0.5 inch from the plant than farther from the plant. These results are not surprising, but the question is why do these symptoms occur at all? Tractor applied fertilizer is spaced 2-3 inches from the plant to avoid fertilizer burn. One possible explanation on how the fertilizer may reach the lettuce roots has to do with soil type. As I mentioned the problem seems to occur on heavier soils; these soils are more prone cracking which can permit liquid fertilizers to flow a short distance towards the seedline during the application. If the material flows close enough to the taproot of the young plant, then it can burn the tap root at the level of injection in the soil. This explanation may explain why the problem occurs at more or less a uniform depth in the soil and why affected plants are scattered in the field (e.g. scattered plants or 2-3 affected plants next to healthy plants) (Photo 4).
Photo 1: Typical symptoms of plants with fertilizer burn on the roots
Photo 2. Plant with the tap root burned of 1.5-2.0 inches down in the soil
Photo 3. Close up of the burned of tap root (note that the remainder of the root tissue is healthy)
Photo 4. Pattern of the problem in the field
- Author: Steven Koike, Plant Pathology Farm Advisor
During the month of April 2011 in coastal California, a number of growers and PCAs are seeing wilting and stunting of young lettuce plants. Affected lettuce typically range in size from the 4-to-6 true leaf through rosette stages and are randomly distributed throughout the field as individually affected plants. Such plants initially fall behind in development and may appear slightly stunted. As the problem worsens, these plants will start to wilt during the day. Eventually the older leaves may turn yellow then brown, all foliage wilts, and the plant can collapse and die. Such a condition is caused by two different factors: ammonium toxicity and abrasion of the crown due to wind damage.
Ammonium toxicity: This problem results from the buildup of ammonium in soils planted with young lettuce seedlings. Ammonium toxicity occurs when soils are cool and the soil surface is sealed or compacted, resulting in slow nitrification rates. This disorder can also occur in fields with poorly drained, waterlogged soils. The use of fertilizers that contain ammonium can contribute to ammonium toxicity. When the lettuce root is injured by the ammonium buildup, the lettuce foliage will show the stunting and wilting symptoms described above.
To make a preliminary diagnosis of ammonium toxicity, examine the entire root system of the young lettuce. The central core of the taproot first turns yellow to light brown, then becomes dark brown to red in color (Photo 1). In severe cases, the central core collapses and a cavity forms throughout the length of the root (Photo 2). Lateral roots may be short, with blackened tips. In some situations the external surface of the root turns yellow or light brown and develops cracks; these symptoms mimic corky root disease. A more thorough analysis will require testing to eliminate the presence of the fungal pathogen Fusarium oxysporum f. sp. lactucae, the causal agent of Fusarium wilt. Because ammonium toxicity is associated with certain environmental and weather conditions, there are no effective management steps that can prevent this disorder.
Crown abrasion or wind-whipping: This problem is the result of physical damage to young lettuce plants. Strong winds will whip the lettuce crown and leaves back and forth, causing the crown to rub against the soil and become injured. These crowns will show characteristic pinching, drying, and collapse of tissue at the soil line of the lettuce (Photo 3), resulting in the stunting and wilting symptoms described above. In severe cases the lettuce plant can be completely girdled, resulting in the stubbing-off of the base of the plant; such plants will not develop to maturity (Photo 4). This “wind-whipping” of lettuce tends to be more severe in fields having coarser, sandier soils. Because wind-whipping is associated with certain environmental and weather conditions, there are no effective management steps that can prevent this disorder.
Not a disease: Neither ammonium toxicity nor wind-whipping is associated with any disease. While above-ground symptoms of both of these disorders may resemble damping-off diseases, lettuce in coastal California is not affected by damping-off pathogens such as Pythium or Rhizoctonia. Corky root disease usually does not result in the distinctive central core discoloration. Verticillium wilt may cause central core discoloration, but this disease only shows up on mature lettuce close to harvest and not on such young plants. The only disease that looks similar to ammonium toxicity is Fusarium wilt, which can infect young lettuce plants. However, Fusarium wilt always occurs in patches in a field and will not occur as individual plants that are randomly scattered throughout the planting.
Photo 1: Discoloration of the central core of the lettuce root is characteristic of ammonium toxicity.
Photo 2: Ammonium toxicity can cause the lettuce root to develop a central hollow cavity.
Photo 3: Wind-whipping results in a pinched, collapsed crown at the soil line of the lettuce plant.
Photo 4: Severe wind-whipping can completely girdle the lettuce crown.