Brown spots on lettuce

Vegetables Produce Facts English

Physiological and Physical Disorders
Many disorders have been identified for iceberg lettuce. Some very common and important disorders are the following:
Tipburn. A disorder caused in the field and is related to climactic conditions, cultivar selection and mineral nutrition. Leaves with tipburn are unsightly and the damaged leaf margins are weaker and susceptible to decay.
Russet Spotting. A common disorder due to exposure to low concentrations of ethylene which stimulates the production of phenolic compounds which lead to brown pigments. Russet spots appear as dark brown spots especially on the midribs. Under severe conditions, russet spots are found on the green leaf tissue and throughout the head. The disorder is strictly cosmetic but makes the lettuce unmarketable. Ethylene contamination may occur from propane fork lifts, transport in mixed loads, or storage with ethylene-generating fruits such as apples, pears and peaches.
Brown Stain. The symptoms of this disorder are yellowish-reddish-brown large, depressed spots on the midribs mostly. These may darken or enlarge with time. Brown stain also appears as reddish-brown streaks in some cases. Brown stain is caused by exposure to above 3% CO2 atmospheres, especially at low temperatures.
Pink rib. A disorder in which the midribs take on a pinkish coloration. Overmature heads and high storage temperatures increase the disorder. Ethylene exposure does not increase the disorder and low O2 atmospheres do not control it.
Breakage of the midribs often occurs during field packing and causes increased browning and increased susceptibility to decay.
Pathological Disorders
Bacterial soft-rots are caused by numerous bacteria species and result in a slimy breakdown of the infected tissue. Soft-rots may follow fungal infections. Trimming outer leaves, rapid cooling and low temperature storage reduce development of bacterial soft-rots.
Fungal pathogens. May also lead to a watery breakdown of lettuce (watery soft-rot caused by Sclerotinia or gray mold rot caused by Botrytis cinerea) but are distinguished from bacterial soft-rots by the development of black and gray spores. Trimming and low temperatures also reduce the severity of these rots.

Disorders Photos

Title: Grey Mold

Photo Credit: Adel Kader, UC Davis

Title: Mechanical Damage

Photo Credit: Adel Kader, UC Davis

Title: Pink Rib

Photo Credit: Adel Kader, UC Davis

Title: Russet Spotting

Photo Credit: Adel Kader, UC Davis

Title: Rusty Brown Discoloration

Photo Credit: Adel Kader, UC Davis

Title: Severe Symptoms of Russet Spotting

Photo Credit: Adel Kader, UC Davis

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

Assume for a minute you are inspecting a load of lettuce. You come across some heads (See image below) that have a distinct discoloration affecting the veins and midribs. One thing you know for certain is the discoloration definitely materially affects the appearance of the head, thus you would score it as a defect.

But what is this defect called? This picture was sent to me from a person training to be a fruit and vegetable inspector in South Korea. Although I only have a few pictures to view, it appears this inspector-in-training may have found lettuce affected with a defect called Rusty Brown Discoloration. The cause of this defect is unknown, but has been found in lettuce grown in Arizona or California. This defect is not commonly found, and I would venture to guess most of the younger USDA inspectors have never seen this defect in their careers. I have seen Rusty Brown Discoloration only a few times during my 30 plus years, and I have yet to come across it during the past 15 years.

Again, because I only have a picture to go by, I am not 100% sure this lettuce is affected by Rusty Brown Discoloration, but the symptoms make it highly likely. The discoloration will affect the midribs and veins and will eventually cover the entire crown. The discoloration spreads quickly on the head, and is always scored as a serious defect. The U.S. No. 1 Grade for Lettuce only allows 6% serious damage defects.

Shown above is the official USDA visual aid depicting Rusty Brown Discoloration. This visual aid was developed in 1970. Please share your thoughts if you have seen this defect recently, as everyone would enjoy hearing the comments associated with this defect.

Why do spots appear on iceburg lettuce during cold storage?(0~4℃)

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If you’ve noticed some brown spots on lettuce you just bought, you might be wondering if it’s safe to eat. You’re not alone. A lot of shoppers are puzzled when they see this discoloration on their “fresh” produce. So, what’s the deal here?

According to the scientific journal Plant Physiology, these strange-looking marks are known as “russet spotting.” A physiological disorder that may develop during postharvest transport and storage, russet spotting is particularly common in iceberg lettuce. According to the Postharvest Center at the University of California, russet spotting occurs because of exposure to low concentrations of a gas called ethylene. This exposure might happen for a variety of reasons, including transportation in mixed loads, or even close proximity to ethylene-generating fruits like apples in storage.

Although russet spotting may not be easy on the eyes, experts say the issue is strictly cosmetic. In other words: Russet spotting alone is no reason to throw your salad in the trash. Phew!

Of course, it’s worth keeping in mind that if you plan to serve salad to a big group of people, lettuce with russet spotting may not look super appealing to your crew. In that case, it may be worth your time to wait until just before the gathering to purchase your lettuce of choice — and inspect all those lettuce heads closely before making your selection.

More importantly, it’s crucial for you to take a close look at your lettuce — and any food, really — before actually serving it to yourself and others. The last thing you’d want is to discover the problem of mold on your produce after it’s already on your plate. Any lettuce that is slimy, decayed, or smelly is also a no-go.

But if all you’re seeing on your lettuce is a few brown spots or streaks, you can feel confident about digging in. Your healthy salad recipes await!

Next, see some of the tastiest superfoods that can help you live longer in the video below:

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Chronic problem. Bacterial leaf spot of lettuce has been affecting coastal California crops for many years and has become a chronic problem. The disease was first noted in California in 1964 and became an economic concern in the 1990s. Bacterial leaf spot now occurs to some degree every season. In addition, it is possible that new strains of the pathogen may cause disease in previously resistant lettuce cultivars. For these reasons researchers are continuing to study the problem and are now requesting samples from cases that occur in 2013.

Symptoms. Early symptoms of bacterial leaf spot are small (1/8 to 1/4 inch), water-soaked spots that usually occur only on the older, outer leaves of the plant. Lesions are typically angular in shape because the pathogen does not penetrate or cross the veins in the leaf. Lesions quickly turn black—this is the diagnostic feature of this disease. 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. If disease is severe, secondary decay organisms (bacteria, Botrytis cinerea) can colonize the leaves and result in a messy soft rot of the plant. Bacterial leaf spot can occur on all types of lettuce: iceberg, romaine, leaf, and butterhead. See photos below.

Pathogen. Bacterial leaf spot is caused by Xanthomonas campestris pv. vitians. The taxonomy of this pathogen is unsettled and the name is likely to change in the next few years. This bacterium is a pathogen mostly limited to lettuce, though under greenhouse conditions several weeds in the same plant family can develop bacterial leaf spot disease when inoculated. We have not yet found naturally infected weeds showing leaf spot symptoms in the field. Some researchers indicate that X. campestris pv. vitians from lettuce can infect very different crops such as pepper and tomato when these plants are artificially inoculated; however, naturally infected pepper and tomato have never been found in California. Bacterial leaf spot disease of lettuce should not be confused with other Xanthomonas diseases. For example, bacterial spot disease of tomato and pepper is caused by a distinct pathovar (Xanthomonas campestris pv. vesicatoria); this pathogen will not infect lettuce. However, a related pathogen caused bacterial leaf spot on radicchio in California.

Disease cycle. 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, though the extent and frequency of seedborne inoculum is not currently known. If lettuce transplants are grown from infested seed, the pathogen may become established on plants during the greenhouse phase of growth. The bacterium can survive for up to five months in the soil. Therefore, infected lettuce plants and residues, once disked into the soil, can supply bacterial inoculum that can infect a subsequent lettuce planting. The bacterium has also been found surviving epiphytically on weed plants, though the significance of this factor is not known. In terms of time of year, a very consistent pattern of bacterial leaf spot outbreaks is documented for the Salinas Valley. There is almost an annual pattern in which severe bacterial leaf spot occurs in August and September. Researchers have not clearly documented why the disease consistently occurs at severe levels in this late summer period.

Control. Clearly the elimination or reduction of the use of overhead sprinkler irrigation will significantly curtail this disease in all situations, except when rains occur. Some resistant lettuce lines have been identified, though resistance is not widely available in currently used cultivars. Residual bacterial inoculum, left in the soil following an infected lettuce crop, will potentially cause problems for the next lettuce planting unless that planting is delayed for five months or longer. Therefore, crop rotation schemes will need to be evaluated if bacterial leaf spot is a chronic problem in fields heavily planted to lettuce. Effective foliar sprays have not been identified for this disease. Lettuce seed should be free of the pathogen.

Samples needed. Differences in pathogen genotypes have been demonstrated and correlated to disease responses on resistant and susceptible lettuce cultivars. In California the deployed lettuce germplasm is resistant to the strains of the pathogen collected many years ago in California. We therefore request samples of bacterial leaf spot disease so as to determine if novel, resistance-breaking strains are found in California. If you encounter this disease, samples can be submitted to the Cooperative Extension Diagnostic Lab in Salinas (1432 Abbott Street, Salinas).

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