- 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.
- Author: Steven T. Koike
White mold disease, caused by the fungus Sclerotinia sclerotiorum, is causing damage to a number of vegetable crops in California and Arizona during the late 2010 and early 2011 months. On the coast of California, white mold is being found on crucifer crops such as broccoli and cauliflower. In the desert regions white mold is causing damage on broccoli, cauliflower, celery, lettuce, and other vegetables (for lettuce this disease is commonly called lettuce drop). White mold incidence on these crops appears to be greater than normally observed. See photos 1 through 6 below.
The first symptoms on most vegetable crop hosts are small, irregularly shaped, water-soaked areas on stems, leaves, pods, or flower heads. These infections quickly develop into soft, watery, pale brown to gray rots. Rotted areas can expand rapidly and affect a large portion of the plant. Diseased tissues eventually are covered with white mycelium, white mycelial mounds that are immature sclerotia, and finally mature, hard, black sclerotia. Mature sclerotia usually form after tissues are rotting and breaking down. Plants with infections on the main stems can completely collapse and fall over.
The black sclerotium is the survival stage of the fungus and can measure from ¼ to ½ inch long. Sclerotia are found in the soil and can directly infect plants if stems are in close proximity. However, these winter cases of white mold are due to ascospore infections. If sufficient soil moisture is present, shallowly buried sclerotia germinate and form small, tan mushroom-like structures called apothecia (photos 7 and 8). Ascospores (photos 8 and 9) are released from apothecia and carried by winds to the host plant. These ascospores are responsible for these winter infections and result in disease of the above-ground parts of plants. The relatively cool, moist weather found in most regions has allowed for the production of apothecia production and ascospore releases.
For ascospores to start colonizing plant tissues, nutrients and plant fluids from damaged tissues are usually needed. This is why white mold is very severe if ascospores land on compromised tissues such as lettuce leaves with tip burn, leaves and heads damaged by frost or other factors, stems with open wounds or exposed leaf traces (vascular tissue in the stem that is left exposed when a lower leaf falls off), and senescent leaves and stems.
Controlling white mold under these winter weather conditions is difficult. Protective fungicides provide some assistance and can be used effectively in lettuce. However, such fungicides need to be applied prior to ascospore flights and usually will require multiple sprays. Fungicides may not be warranted for crucifer crops.
Steve Koike thanks Jeff Rollins and Karen Chamusco for assistance with photographs for this article.
Photo 1: White mold (lettuce drop) on romaine lettuce.
Photo 2: White mold (lettuce drop) on romaine lettuce, showing white mycelium and two black sclerotia.
Photo 3: White mold on broccoli stems.
Photo 4: White mold on broccoli stem, showing white mycelium and one black sclerotium (center).
Photo 5: White mold on cauliflower head, showing white mycelium.
Photo 6:White mold on celery, showing numerous black sclerotia.
Photo 7: One sclerotium and several apothecia (spore producing structures) of Sclerotinia sclerotiorum.
Photo 8: Microscopic view of the spore-producing apothecium of Sclerotinia sclerotiorum. Note the lined-up ascospores (red) ready to be released. Photo used by permission (K. Chamusco).
Photo 9: Microscopic view of ascospores lined-up in a tube (called an ascus) and ready to be released. Photo used by permission (J. Rollins).
- Author: Steven T. Koike
As the coastal California lettuce crop heads into the ending fall season and as the inland county region initiates its fall lettuce cycle, both crops are being affected by Tomato spotted wilt virus (TSWV). Diseased plants have leaves with irregularly shaped, brown to dark brown lesions and dead (necrotic) areas (Photo 1); this necrotic tissue can resemble burn damage caused by pesticide or fertilizer applications. Chlorosis (yellowing) can also be observed. Depending on the age of the plant when first infected, these necrotic and chlorotic symptoms can occur on both the older, outer foliage as well as the younger, inner leaves. If plants are affected with TSWV early in their development, growth may be severely stunted. The virus is vectored by thrips and in California is primarily spread by the western flower thrips (Frankliniella occidentalis) (Photo 2).
All lettuce types are susceptible, and TSWV has been identified in iceberg, butterhead, romaine, and leaf lettuces. TSWV has an extremely wide host range that includes over 500 crop and weed species. Vegetable crop hosts include basil, bean, celery, cucumber, eggplant, endive, escarole, fava bean, lettuce, pea, pepper, potato, radicchio, spinach, and tomato. This host range may explain, in part, why TSWV has been observed in a number of lettuce fields in the San Joaquin Valley. The relatively cool summer temperatures have resulted in delayed tomato harvests, causing an overlap of the summer tomato and fall lettuce crops. Thrips vectoring TSWV are therefore able to readily move from the late tomato plantings and into the lettuce fields. (For related information see research conducted by the Gilbertson team (UC Davis) and sponsored by the California Processing Tomato group.)
Growers and pest control advisors should exercise caution if attempting to identify TSWV in the field and without testing. In the coastal region, symptoms caused by the very closely related, thrips-vectored Impatiens necrotic spot virus (INSV) are identical to those caused by TSWV. Romaine and leaf lettuces are susceptible to the lettuce dieback virus complex (Lettuce necrotic stunt virus [LNSV] and Tomato bushy stunt virus [TBSV]) which is also common on the coast.
Photo 1: Tomato spotted wilt virus (TSWV) on lettuce.
Photo 2: Western flower thrips, vector of TSWV.
- Author: Steven T. Koike
- Author: Richard Smith
In the months of July and August of 2010, a number of vegetable crops in California’s central coast were affected by calcium deficiency disorders. Usually referred to as “tipburn” (except in the case of celery, for which the disorder is called “blackheart”), these problems are often triggered by environmental factors and hence can affect a large number of acres and plantings during windows of time when tipburn is occurring in Salinas and other coastal valleys.
Tipburn symptoms on lettuce, endive, radicchio, and spinach are distictive. Symptoms occur on the margins of developing leaf tips and initially consist of light to dark brown lesions and necrosis. In romaine, tipburn is often first seen on the small veins along the margin of young leaves. In severe cases tipburn can progress and result in extensive damage to these leaf margins. Symptomatic leaves are usually found within the inner whorls of open head vegetables and underneath the enclosing wrapper leaves of closed head vegetable types. For spinach, tipburn always affects the inner, newly developing leaves which may develop a hooded appearance as the leaf continue to expand around the dead tissue. For blackheart of celery, symptoms form on the margins of developing leaf tips deep within the central growing point. Such symptoms consist of light to dark brown speckling, lesions, and necrosis. As the celery grows, the damaged tissues turn black and the affected foliage grows up and out of the inner plant whorl.
For tipburn of artichoke, the margins of immature leaves turn black as in celery. In addition, the immature artichoke flower buds develop black lesions along the upper tips and edges of flower bracts. Cauliflower also develops tipburn, with the inner wrapper leaves enclosing the cauliflower head turning tan to light brown.
Tipburn is a localized calcium deficiency. It often develops along the margins of leaves in the final weeks before harvest when the plant growth rate is at its highest. It is often unrelated to soil calcium levels. Because calcium is not very mobile within the plant, expanding tissues on young leaves and growing points may run short of this essential element and begin to develop deficiency symptoms. Conditions that favor rapid plant growth (warm temperatures and high fertilization rates) or low transpiration rates (foggy conditions) may create conditions that trigger this disorder.
Managing this disorder is difficult. Varieties vary in susceptibility, but tipburn resistance is only available in head lettuce. Maintaining even soil moisture levels and preventing the root zone from drying out helps increase calcium uptake and may reduce the risk or severity of tipburn. For field grown vegetables, soil and foliar calcium supplements have not provided consistent control.
For further information on tipburn see Monterey County Crop Notes - July/August 2007
Initial symptoms of tipburn on romaine occur on veins at the edge of the leaf.
Tipburn of butter lettuce.
Tipburn of cauliflower.
Blackheart of celery.
Tipburn of artichoke.