The unusually cold, wet, and rainy weather during March through June 2011 has created conditions that favor the development of Botrytis crown rot of lettuce in several counties in coastal California. Botrytis crown rot, also known as gray mold, has resulted in stand loss and reduced yields in numerous fields.

Transplanted lettuce: Crown tissue on transplants in the field becomes brown to orange-brown in color and soft. The characteristic fuzzy gray sporulation of the pathogen is usually present on the affected crown tissue in contact with the soil. Botrytis crown rot causes transplanted lettuce to wilt, collapse, and eventually dry up and die. While still in trays and under greenhouse conditions, lettuce transplants can also become infected with gray mold; however, such infections are usually associated with old, overgrown transplants in which the older leaves have begun to age, turn yellow, and decline (senesce).

Direct seeded lettuce: Direct seeded lettuce can also succumb to Botrytis crown rot. Under most conditions such fields have lower disease incidence and the problem is less serious. However, in spring 2011 a number of direct seeded fields also experienced significant dieback from this disease.

 

Mature lettuce: Lettuce plants near maturity may appear healthy but develop crown rot close to harvest. Diseased plants will start to wilt, lower leaves turn yellow then brown, and the entire plant will collapse. Examination of the crown will show extensive gray sporulation and the soft, orange-brown decay. Such plants likely were infected earlier in the season and collapsed when the pathogen rotted a significant portion of the lettuce crown.

The pathogen. The causal agent of gray mold on lettuce is Botrytis cinerea. This fungus is a very common organism that readily grows as a saprobe on dead, declining plant tissue and organic matter. The characteristic fuzzy, velvety, grayish brown growth of the fungus can often be readily seen on diseased areas of the lettuce, especially on lettuce crowns in contact with soil and that may be shielded from the sun by overlying leaves. Black sclerotia (hard fungal resting structures measuring from 1/8 to 1/4 inch in diameter) may form on these diseased tissues, although some isolates produce few or no sclerotia. Sclerotia are usually dome-shaped or rounded and may appear similar to sclerotia produced by the species of Sclerotinia (S. sclerotiorum) that produces large sized resting structures. Botrytis cinerea of lettuce is the same pathogen that causes gray mold disease on grape, strawberry, tomato, ornamental plants, and many other crops.

Disease factors: Botrytis crown rot affects all types of lettuce: iceberg, butterhead, green leaf, red leaf, romaine. Botrytis cinerea most readily infects lettuce tissues that are damaged and exposed to moist, wet conditions. For this reason, Botrytis crown rot is most commonly seen on transplanted lettuce. The process of transplanting lettuce results in unavoidable, minor cracks and injuries to the transplant. If B. cinerea inoculum is lacking, such injuries are incidental and cause little concern. However, if the fungus is present then such wounds allow the pathogen to readily invade and colonize the plant crown. Because spring and early summer romaine is often transplanted, most Botrytis crown rot cases in 2011 involved romaine.

Diagnosis: Diagnosing gray mold will require careful examination. Overall plant wilting and collapsing symptoms caused by gray mold may look very similar to such symptoms caused by Sclerotinia minor (lettuce drop) and perhaps Phoma exigua (Phoma basal rot). Accurate diagnosis, therefore, requires careful examination of the crown and perhaps lab confirmation. Note that if rotted lettuce crowns are colonized by white fungal mycelium, the pathogen is likely Sclerotinia.

Control. Because B. cinerea initiates infection on damaged tissues, as much as possible minimize damage to lettuce that is caused by cultural practices, environmental extremes, or other pathogens and pests. Use transplants that are not too large and overly mature; older transplants are subject to additional leaf breakage and damage during planting, and hence are more susceptible to gray mold infection. Limit damage to lettuce transplants during the planting process, though it is not possible to prevent all injury. In the field, reduce leaf wetness by avoiding or reducing sprinkler irrigation. Schedule crop residue incorporation and soil preparation so that excessive plant residues are minimal at planting. It may be helpful to apply fungicides to protect plants from gray mold. However, if weather conditions strongly favor Botrytis crown rot, such applications may be limited in effectiveness. Before using any fungicide for the control of B. cinerea on lettuce, check product labels and your local Agricultural Commissioner's Office for use information and restrictions.

 

Figure 1. Orange-brown soft rot and gray sporulation on lettuce caused by Botrytis cinerea

Figure 2. Orange-brown soft rot and gray sporulation on lettuce caused by Botrytis cinerea.

 

Figure 3. Reduced romaine stand due to Botrytis crown rot of transplants.

 

Figure 4. Lettuce transplant infected by Botrytis cinerea.

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

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.

Since 2002, a severe leaf spot disease on parsley has occurred throughout central coastal California and particularly in Monterey County. Three different bacterial pathogens (Pseudomonas syringae pv. apii, P. syringae pv. coriandricola and an organism very closely related to P. viridiflava) have been associated with these outbreaks on parsley. Of interest to researchers and of potential importance to growers is the fact that two of these bacteria were already causing problems in coastal crops. Pseudomonas syringae pv. apii is the causal agent of northern bacterial blight of celery and P. syringae pv. coriandricola causes bacterial leaf spot of cilantro. Symptoms of all three diseases are similar and consist of small (usually less than ¼ inch in diameter) leaf spots that are noticeably angular in shape, with the edges of the spot restricted by leaf veins. The color of the leaf spots can vary from light tan to brown to dark brown. These bacterial leaf spots penetrate the entire leaf, so that the spot will be visible from both the top and bottom sides of the infected tissue (in contrast to chemical damage or abrasion in which the symptom is usually only seen from the top side of the leaf). See photos below.

Our research team is also investigating a possible new bacterial disease on fennel, as well. Because of these developments on commercially grown plants in the Apiaceae, we are seeking additional samples of foliar problems from any member of the Apiaceae crop group: celery, cilantro, dill, fennel, parsley, and others. Further clarification of the relationship between these various bacterial pathogens, determination of which hosts are susceptible to which pathogen, and other aspects may assist industry in managing these diseases.

The best samples will consist of diseased plants collected from several different locations of a field. Send samples to the UC Cooperative Extension diagnostic laboratory in Salinas: 1432 Abbott Street, Salinas CA, 93901 (phone 831-759-7550), attention Steve Koike.

 

Bacterial leaf spot of celery.

 

Bacterial leaf spot of cilantro.

 

Bacterial leaf spot of parsley.