Beginning in early April, 2012, the UC Cooperative Extension diagnostic lab in Salinas began to receive lettuce samples exhibiting obvious symptoms of a virus problem. Samples continued to be submitted throughout the month. All samples tested positive for the thrips-vectored Impatiens necrotic spot virus (INSV). This virus first began to cause damage to California lettuce in 2006. Since that time, INSV has occurred to a greater or lesser degree every season. However, significant INSV outbreaks usually are found beginning in late May or early June. Seeing INSV on lettuce in April is therefore unusual and growers and pest control advisors should carefully monitor these situations.
INSV-infected plants have leaves with brown to dark brown spots and dead (necrotic) areas; this necrotic tissue can resemble burn damage caused by pesticide or fertilizer applications (see photos below). Affected leaves can be distorted and twisted. Extensive necrosis can cause much of the leaf to become brown, dry, and dead. Some leaf yellowing can also be observed. Yellowing and the brown spotting tend to be observed on the newer leaves near the center of the plant’s growing point. If plants are affected with INSV early in their development, growth may be stunted. All lettuce types are susceptible, and INSV has been confirmed on iceberg, butterhead, romaine, and leaf lettuces. INSV can infect many other crops and weed species; the virus is vectored by the Western flower thrips (Frankliniella occidentalis).
Growers, PCAs, and other field personnel should exercise caution in diagnosing INSV disease because the lettuce dieback viruses cause very similar symptoms (see table below). Lettuce dieback is caused by two pathogens: Lettuce necrotic stunt virus (LNSV) and Tomato bushy stunt virus (TBSV). Infected lettuce can be severely stunted, especially if infected early in plant development. The oldest, outer leaves can be severely yellowed. Brown, necrotic spots and lesions later develop in these outer leaves. The younger, inner leaves remain dark green in color, but can be rough and leathery. LNSV/TBSV infects only romaine, butterhead, and leaf lettuces; modern cultivars of iceberg lettuce are immune. The LNSV/TBSV virus complex is a soilborne problem and no vector (insect, nematode, fungus) is known to spread these viruses.
Photos: Impatiens necrotic spot virus (INSV) on lettuce.
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.
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.