- Author: Steven T. Koike
Weather and lettuce diseases. Late spring rains, cold temperatures, and high humidity are making it possible for two important foliar diseases of lettuce to show up in California and cause damage in 2012. Both bacterial leaf spot and anthracnose have been observed in numerous fields throughout the central coast. If rains continue or if the crop is irrigated with sprinklers, both diseases can result in significant damage to lettuce leaves and resulting loss of yields due to poor quality of the harvested heads. Experienced field personnel will likely recognize these two problems without difficulty; however, we recommend that if there is any question, laboratory tests be conducted to confirm diagnosis.
Bacterial leaf spot. Initial symptoms are small (1/8 to 1/4 inch), water-soaked spots that occur on the older, outer leaves of the plant. Lesions are typically angular in shape and quickly turn black (photo 1)—this is the diagnostic feature of this disease and readily separates this from anthracnose (which causes white to light pink lesions). If disease is severe, numerous lesions may coalesce, resulting in the collapse of the leaf. Older lesions dry up and become papery in texture, but retain the black color. Lesions rarely occur on newly developing leaves.
Bacterial leaf spot is caused by bacterium Xanthomonas campestris pv. vitians. The pathogen is highly dependent on wet, cool conditions for infection and disease development. Splashing water from overhead irrigation and rain disperses the pathogen in the field and enables the pathogen to infect significant numbers of plants. The pathogen can be seedborne and is introduced into the field via contaminated seed. In addition, the bacterium can survive for up to five months in the soil. Therefore, infected lettuce crops, once disked into the soil, can supply bacterial inoculum that can infect a subsequent lettuce planting.
Spray applications are not effective at managing bacterial leaf spot of lettuce. The disease is managed by using uninfested seed, irrigating the crop via furrow or drip, and avoiding back-to-back lettuce rotations if the first crop was diseased.
Anthracnose. Early symptoms are small (1/8 to 1/4 inch), water-soaked spots occurring on outer leaves. Spots enlarge, turn yellow then tan, and are usually angular in shape. Under cool, rainy conditions, white to pink spore masses of the fungus will be visible in the centers of the tan colored lesions (photo 2)—this is the diagnostic feature of this disease and readily separates this from bacterial leaf spot (which causes black lesions). If disease is severe, the lesions will coalesce and cause significant dieback of the leaf and in some cases will result in stunting of the plant. As spots age, the affected tissue will dry up and become papery in texture. Eventually the centers of these spots can fall out, resulting in a shot hole appearance. Anthracnose lesions are often clustered along the midribs of lower leaves.
Anthracnose is caused by the fungus Microdochium panattonianum. This fungus infects only lettuce and does not cause disease on any other crop. The fungus can survive for up to four years as microsclerotia in soil. The anthracnose pathogen requires cool, wet conditions for infection and symptom development and hence is associated with rainy weather. Splashing water moves microsclerotia and conidia from soil onto leaves, resulting in infection.
To manage anthracnose, first avoid planting lettuce in fields having a history of the disease. Use irrigation systems (furrow or drip irrigation) that reduce or eliminate splashing water and leaf wetting. Apply protectant fungicides, such as strobilurins, which are effective for controlling this disease.
Photo 1. Bacterial leaf spot results in black, angular shaped lesions on lettuce leaves.
Photo 2. On lettuce, anthracnose results in tan, angular shaped lesions that are covered with the white to pink growth of the fungus.
- Author: Steven T. Koike
Experienced growers, pest control advisors, and other field professionals involved with broccoli already know that the winter period can signal increased problems due to head rot (also known as pin rot). Favored by cool temperatures and prolonged periods of moisture from rain, dew, and fog, broccoli head rot continues to be a damaging and yield-reducing factor because preventative measures have yet to be consistently effective. Spray applications have not proven to be consistently reliable tools to prevent head rot. While some cultivars (especially those having rounded, dome-shaped heads) may be less susceptible to head rot, true resistance has not been demonstrated for cultivars grown in California. Trying to avoid the use of overhead sprinkler irrigation is apparently the only cultural practice that helps reduce disease; however, the winter and early spring weather will enhance head rot even if growers use drip or furrow irrigation.
Field personnel should remember that two types of head rot affect the crop in California. For bacterial head rot, initial symptoms on the immature broccoli heads consist of a water-soaked or greasy discoloration of the surfaces of small groups of the unopened flowers. Later, the affected portions of the head turn brown to black and the infection spreads and affects larger parts of the head. The tissue becomes soft and gives off a very bad odor. For bacterial head rot there will not be any fungal growth unless secondary molds colonize and cause further decay.
The second type of head rot is Alternaria head rot. For this fungal problem, early symptoms consist of a water-soaked discoloration that later turns dark brown to black. Tissues infected with Alternaria are usually not as soft and smelly as heads infected with the bacterial pathogens. Alternaria readily produces dark green spores on the diseased head tissue. Secondary molds and bacteria cause further decay.
Photo 1: Bacterial head rot of broccoli
Photo 2: Bacterial head rot of broccoli
Photo 3: Alternaria head rot of broccoli
Photo 4: Alternaria head rot of broccoli
The rainy and cold spring weather in 2010 is apparently having an effect on head lettuce quality in Salinas Valley fields. Symptoms appear as very small, brown flecks and spots along the margins of young leaves (photo 1). Affected leaves are usually found deep within the head. It appears that these defects are occurring in multiple iceberg cultivars in various parts of the valley. Clearly this is a physiological disorder and superficially looks similar to russet spot; however, most of the flecks do not occur on the leaf midribs (photo 2) as would be typical for russet spot. Russet spot is caused by ethylene production and can occur in mature to over mature lettuce, especially following anaerobic conditions in the field. However, in this instance, the location of the flecking along the margin of the leaf more closely indicates tipburn.
The extensive nature of the problem (from Salinas to San Ardo) and the occurrence across varieties indicates that a large-scale factor like weather could be the cause. The heavy rain on April 5 followed by cloudy, cool weather may account for the currently wide distribution of the problem. We are conducting further investigations to more closely determine the cause of this problem.
These defects are not caused by any plant pathogen. Extensive testing has shown that bacterial leaf spot, anthracnose, or other lettuce disease is not associated with these brown flecks. Bacterial leaf spot (caused by Xanthomonas campestris pv. vitians), however, is common this year and should not be confused with this physiological disorder. Bacterial leaf spot occurs on the outer leaves and results in large, black, angular lesions (photo 3).
Photo 1: Lettuce defects in 2010
Photo 2: Typical russet spotting of lettuce
Photo 3: Bacterial leaf spot of lettuce