- Author: Steven T. Koike, Plant Pathology Farm Advisor
Speck on coastal tomato. In summer (May through July) 2011, bacterial speck disease of tomato is commonly found in a number of fields in coastal tomato-growing areas in Santa Clara and Monterey counties. No doubt the periodic and prolonged rains earlier this year are a major factor in this unusual development. Symptoms consist of dark brown to almost black spots on leaves and sometimes stems. Leaf spots can be circular or angular in shape and individual spots are generally smaller than ¼ inch in diameter. Spots are visible from both top and bottom sides of the infected leaf and are sometimes surrounded by a yellow halo. Leaf spots can merge together and result in the death of large areas of the leaf. On occasion the leaf spots may be located along the edges of the leaves and result in elongated lesions. Stems and petioles also can develop dark brown to black spots that are irregular in shape but tend to be slightly elongated along the axis of the stem. If the pathogen is splashed onto the fruit, disease symptoms on green fruit will consist of small (generally less than 1/8 inch in diameter), slightly raised, superficial black lesions or specks. Such specks usually are not surrounded by haloes.
Confusion of symptoms. Symptoms of bacterial speck on tomato foliage can closely resemble symptoms from the other bacterial leaf spot disease of tomato, bacterial spot (caused by Xanthomonas campestris pv. vesicatoria). In addition, Tomato spotted wilt virus may infect tomato and also cause circular to irregularly shaped, dark brown spots on leaves, again resembling bacterial speck. For all three of these problems, precise confirmation of the pathogen will require laboratory testing.
The speck pathogen. Bacterial speck is caused by the pathogenic bacterium Pseudomonas syringae pv. tomato. Strains of this pathogen are thought to be host specific to tomato, and two distinct races (races 0 and 1) have been documented. This pathogen is seedborne. Bacterial speck on the coastal tomato crop is usually quite minor in severity; once rains stop and if overhead sprinklers are not used, the disease is rarely seen in the summer.
Disease development. Primary inoculum can come from infested seed, diseased plant debris in the soil, or infected volunteer tomato plants. Infested seed is a particularly important inoculum source; if infested seed is used to produce transplants, these plants are often grown under greenhouse conditions that encourage disease development and spread. The practice of using overhead sprinkler irrigation (in the greenhouse or in the field) can significantly spread the pathogen. Once diseased transplants are in the field, the pathogen can be spread plant-to-plant via splashing water (rain, sprinkler irrigation), contaminated tools and implements, and worker’s hands. Disease development is favored by high humidity (greater than 80% relativity humidity), free moisture, and relatively cool temperatures. Again, the rains that occurred through June 2011 certainly encouraged the development of this problem. Interestingly, for all the numerous tomato samples tested by the UCCE diagnostic lab in Salinas, all samples were positive for speck only; bacterial spot disease was not detected at all thus far in 2011.
Control. Obtain and plant high quality seed that does not have detectable, economically important levels of P. syringae pv. tomato. Use a hot water seed treatment or treat seed with hydrochloric acid, calcium hypochlorite, or other recommended materials. Seed treatments must be applied carefully because hot water treatments can reduce seed viability and germination. Avoid using overhead sprinkler irrigation in the field. Applying preventative spray applications to transplants still in trays or to field planted crops may be appropriate. Before using any pesticides, check with your local Agricultural Commissioner's Office and consult product labels for current status of product registration, restrictions, and use information.
Photo 1. Oval to rectangular leaf spots caused by bacterial speck disease of tomato.
Photo 2. Angular spots on leaf margin caused by bacterial speck disease of tomato.
Photo 3. Angular spots on leaf margin caused by bacterial speck disease of tomato.
Photo 4. Elongated stem lesions on transplants caused by bacterial speck disease of tomato.
Photo 5. Fruit speck symptom caused by bacterial speck disease of tomato.
- Author: Steven T. Koike
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.
In spring and summer of 2010, plants in some tomato and pepper fields in coastal California have developed virus-like symptoms. Affected plants were found in both Monterey and San Benito counties, though disease incidence was low. Samples submitted to the Gilbertson lab at UC Davis tested positive for the curly top virus, indicating that these symptoms were due to curly top disease.
Curly top can be a severe disease for both tomato and pepper. Symptoms of the disease vary depending on the host plant infected and the stage of growth when infections occur. In general, affected plants show some degree of stunted and distorted growth and leaf curling and light green to yellow discoloration. In tomato, curly top symptoms begin with leaves showing light green coloration, up-curling, and vein purpling on the underside of leaves. Plants become stunted with twisted, distorted, and yellow leaves. Plants infected at an early stage of development stop growing and die, often standing out among nearby healthy plants. Plants infected at later stages of development show distorted growth and light green-yellow leaves with vein purpling; these symptoms will develop on the upper part of the plant or on entire shoots, but the plants generally do not die. Any fruits produced will be stunted and ripen prematurely.
In peppers, plants are stunted with shortened internodes. Leaves are light green or yellow in color, show strong up-curling, and are thick and brittle. Plants infected at a young stage of growth may die, whereas older plants are stunted and leaves again curl upwards and are light green to yellow; any fruits that develop are small, wrinkled, and unmarketable. As for all diseases caused by viruses, confirmation of curly top disease requires laboratory testing because other tomato and pepper viruses can cause symptoms that are similar to those caused by the curly top viruses (e.g., symptoms caused by Tomato spotted wilt virus).
In California curly top disease affects many plants including tomato, pepper, bean, pumpkin, squash, spinach, sugar beet, other crops, and many weeds. Researchers have discovered that curly top disease in the western USA is actually caused by several distinct virus species. Therefore, curly top of tomato or pepper could be caused by one or more of the following viruses: Beet curly top virus (BCTV), Beet mild curly top virus (BMCTV), Beet severe curly top virus (BSCTV). For example, samples in 2008 from Monterey County tested positive for either BMCTV or BSCTV.
The curly top viruses are vectored by the beet leafhopper (Circulifer tenellus). The virus is not carried in seeds nor is it mechanically transmitted (i.e., by touch or physical contact). Disease incidence and distribution in tomato and pepper fields in a given year are dependent on the populations of the beet leafhopper and the migratory feeding patterns of this insect. Most infections tend to occur early in the growing season, but late season infections may also occur, as in 2010. In general, for coastal California, disease incidence is usually low and symptomatic plants occur randomly in a field, indicating where the fast moving leafhopper has stopped to feed. The random, scattered distribution of diseased plants reflects the fact that tomato and pepper are not preferred hosts of the beet leafhopper and that the insects move on in search of preferred hosts, such as plants in the beet family (Chenopodiaceae).
Tomato infected with curly top viruses: If infected early, plants can be severely stunted.
Tomato infected with curly top viruses: Leaves can be thickened, deformed, yellowed, and rolled upwards. Symptomatic tomato leaves often develop purple veins.
Pepper infected with curly top viruses: If infected early, plants can be severely stunted.
Pepper infected with curly top viruses: Leaves can be thickened, deformed, and rolled upwards.