Editor's Note: Wet weather is generally welcome in the arid West for with it comes irrigation water supplies. However, a welcome wet winter and spring like this year also bring on diseases. Steve Koike, Monterey County plant pathology farm advisor and Richard Michelmore, from University of California's vegetable crops department, provided the following information of two of the more common lettuce problems, lettuce anthracnose and downy mildew, associated with wet weather.
Anthracnose disease of lettuce only occurs in California during cool spring seasons when there are significant rains.
Due to the rainy weather pattern of 2005, anthracnose is now developing in a number of lettuce growing areas. Initial symptoms are small (1/16 inch in diameter), water-soaked spots occurring on outer leaves. Spots enlarge, turn yellow, and are usually irregular and angular in shape. Under cool, moist conditions, white to pink spore masses of the fungus will be visible in the centers of the tan-colored 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 and hence the name. Anthracnose lesions are often clustered along the midribs of lower leaves.
Romaine cultivars, in particular, exhibit severe disease along leaf midribs. If infected early and severely, young lettuce seedlings can be killed by anthracnose.
The pathogen is Microdochium panattonianum. Older references give the pathogen name as Marssonina panattoniana. Microdochium panattonianum infects lettuce and other Lactuca species and has also been reported on chicory and endive. At least 5 different races of the pathogen have been identified. The fungus forms tiny, resilient survival structures (microsclerotia) that can persist for up to four years in soil. The anthracnose pathogen requires cool, wet conditions for infection and symptom development… Splashing water moves microsclerotia and conidia from soil onto leaves, resulting in infection. Optimum temperatures for disease development are approximately 60 to 68 degrees and symptoms can appear 4 to 8 days after infection.
Field history key
To prevent disease development, avoid planting early spring lettuce in fields having a history of the disease. Rotate with non-host plants to help reduce soil inoculum levels, though such rotations will not eliminate the pathogen unless hosts are not planted for over four years. Use irrigation systems (furrow or drip irrigation) that eliminate leaf wetting. Resistant cultivars are not widely available.
Protectant fungicides, such as Quadris, are effective for controlling this disease.
Rainy weather last winter and this spring has been blamed for severe outbreaks of lettuce downy mildew in various winter lettuce growing regions (southern California; Huron, Yuma, Arizona). Because the pathogen is favored by leaf wetness, cool conditions, and high humidity, this epidemic is not surprising. Downy mildew is probably the most important foliar disease of lettuce worldwide and can attack all lettuce types.
Losses are experienced when severe downy mildew causes lowered yield and quality and when diseased leaves need to be trimmed from the harvested produce. Infected heads also have lower post-harvest quality due to infections from secondary pathogens such as soft rot bacteria and the gray mold fungus (Botrytis cinerea).
Downy mildew causes light green to yellow angular lesions. White fluffy spores of the pathogen are visible primarily on the undersides of these lesions. With time the lesions turn brown and dry up. Older leaves are in contact with the soil can become soft and rotted due to secondary decay organisms, such as Botrytis cinerea.
On very rare occasions the pathogen can cause systemic infections that result in dark discoloration and streaking of internal vascular and pith tissues. If downy mildew infects the cotyledons of young seedlings, the plants can die.
Greenhouse grown lettuce transplants can also be infected. Cultivated lettuce is the main host, though B. lactucae has been reported to infect wild Lactuca species, artichoke, and related plants in the Asteraceae plant family. However, the pathogen is highly specialized to its host and strains infecting cultivated lettuce are restricted to lettuce.
Downy mildew is caused by the pathogen Bremia lactucae, which can only grow on living plant tissue and belongs in the oomycete group of organisms. The population structure of Bremia lactucae is complex and consists of multiple races (pathotypes) and two mating types (B1 and B2).
There is considerable variation in the ability of isolates of B. lactucae to overcome resistance genes in lettuce. Pathotypes are identified by testing their virulence on a diagnostic set of lettuce cultivars which has different resistance genes. A pathotype is defined as a group of isolates with a particular pattern of virulence that is observed in multiple locations and over multiple years. The worldwide nomenclature of isolates is complicated because different groups use their own numbering systems (for example: NL1, NL2, etc. in the Netherlands; IL1, IL2, etc. in Israel; CA V, CA VI, etc. in California), reflecting the local variation. A standard numbering system is now being used in Europe in which races are assigned BL (for B. lactucae) numbers. In California eight races have been defined over the past 25 years. Pathotypes CA VI, VII, and VIII are currently the most common.All these isolates belong to the B2 mating group.
Humid, cool conditions are required for B. lactucae to sporulate and to infect lettuce; therefore, the disease is usually more problematic in the coastal areas than the desert regions. Initial inoculum consists of conidia from surrounding plants (most likely other lettuce fields). The conidia are produced on lesions at night and are released into the air in the early morning if relative humidity is sufficiently high. The spores are dispersed by winds and are short-lived. Infection takes place in 3 or 4 hours if there is free moisture on the leaves, or if near saturation conditions are present and temperatures are optimal (52-68 degrees). In Europe and elsewhere the sexual survival spore, the oospore, may be a source of initial infection from the soil; however, there is no evidence of this factor being important in California.
The most effective means of controlling downy mildew is to grow resistant cultivars. Some cultivars are resistant to most isolates of B. lactucae currently in California. However, such resistant cultivars are not available for all areas and seasons.
Breeding for downy mildew resistance has been the major focus of seed companies and the number and diversity of cultivars with resistance genes effective against all Californian isolates of B. lactucae are likely to increase greatly in the next few years. Because the pathogen is highly variable and dynamic, resistant cultivars do not remain resistant indefinitely and are overcome by virulent isolates of B. lactucae. Recent breeding efforts have been aimed at introducing different resistances into different lettuce type (crisphead, leafy, romaine, and butterhead) to slow down the breakdown of resistance and prevent the pathogen spreading from one type to another.
In the absence of resistant cultivars, the alternative is to apply fungicides prior to the development of the disease. Greenhouse grown transplants should especially be protected so that downy mildew is not distributed and brought to the field on transplants. Many isolates in California have developed high levels of insensitivity to the Ridomil fungicide (metalaxyl), and many B. lactucae isolates are at least partially resistant to Aliette (fosetyl-Al). Resistance management strategies dictate that different fungicides should be used in rotation to slow down the development of insensitivity.
Some recently registered products (Reason, Tanos, Acrobat) have been very effective in downy mildew trials and should be useful to the lettuce industry.
Research has been aimed at developing disease prediction models for B. lactucae. Such systems potentially can help farmers reduce the number of fungicide applications made to lettuce; however, a reliable and commercially available model is not yet available.
Culturally, irrigation systems, such as drip irrigation, that reduce leaf wetness and humidity should reduce the likelihood of disease but will not prevent disease when weather conditions are conducive to epidemics.