In the last year we have been called out to look at lettuce fields showing uneven growth. The fields looked like they were not growing normally due to a cultural practice. The lettuce plants in the field vary in size with normal plants interspersed with stunted plants (Photos 1 and 2). The patterns in the field do not fit with sprinkler patterns or mechanical issues because affected plants are more randomly dispersed. The pattern may fit with soil insect issues, but no soil insects were present in the affected fields. Sometimes stunted plants were more prevalent in areas that might have poor drainage. The randomness of affected plants is similar to some of the nitrogen toxicity issues that we have detected in some fields (http://ucanr.org/blogs/blogcore/postdetail.cfm?postnum=4931). In some cases the taproot of the plant is missing (a typical symptom of nitrogen toxicity), but taproots are not consistently missing and therefore the symptoms do not fit nitrogen toxicity. Upon close examination of the root system, small necrotic sections of the root are present and we were able to confirm that the lettuce was affected by black root rot disease.
Root symptoms of black root rot of lettuce consist of numerous dark brown to black bands and lesions on small feeder and larger secondary roots (Photos 3 and 4). Internal vascular discoloration (as seen with Verticillium wilt) is absent. If severe infections occur early in the crop cycle, the lettuce seedlings will have damaged taproots and experience delayed growth, resulting in stunted, small plants that otherwise may look healthy. The stunting is usually most evident from rosette stage through crop maturity. In advanced cases, the lower leaves of the plant may turn yellow or even brown. In the Salinas Valley, black root rot has been seen to cause stunting and resulting crop loss on both iceberg and romaine cultivars. Black root rot of lettuce has been known for many years in Australia and other regions. The disease was first documented on lettuce in California in 2005. Our diagnostic lab has periodically received black root rot cases since then.
Black root rot is caused by the soilborne fungus Thielaviopsis basicola. The fungus makes two types of spores, with one type being a thick walled spore (Photo 5) that resists weathering and enables the fungus to survive for long periods in the soil. This pathogen is reported to have a broad host range that includes the following vegetable crops: bean, beet, carrot, garlic, lettuce, onion, pea, pepper, tomato. In coastal California we have not found black root rot on most of these crops; black root rot disease has been confirmed only for lettuce and tomato.
Differences in strains or isolates of T. basicola may exist. For example, researchers found that when inoculated onto lettuce and bean, T. basicola isolates from lettuce were very aggressive and caused significant damage. However, the same lettuce isolates caused very little damage to other known hosts. Such research indicates that T. basicola may consist of isolates having some host preferences.
Industry-wide, black root rot of lettuce is a very minor problem that apparently only affects a few fields. However, growers and PCAs should be aware of this disease and notify Farm Advisors if black root rot becomes more common. For other crops, black root rot is managed by a combination of resistant cultivars, crop rotation to non-hosts, sanitation practices so as to not spread contaminated soil/mud to uninfested fields, and application of fungicides to seedlings.
Photo 1. Typical pattern of Thielaviopsis basicola infection in lettuce.
Photo 2. Stunted lettuce affected with black root rot (on left)
Photo 3: Diseased roots of lettuce affected by black root rot.
Photo 4. Close-up of black root rot lesion on fine feeder roots of lettuce.
Photo 5: Dark conidia of Thielaviopsis basicola forming on lettuce root.
- Author: Jim Correll, University of Arkansas Steven Koike, University of California Cooperative Extension
Another new race, the 13th, of the downy mildew pathogen (Peronospora farinosa f. sp. spinaciae) of spinach has been found and documented. First identified in January 2010 from spinach in Holtville, California, this race breaks the resistance of several important cultivars. The isolate was initially designated as UA0510C and was characterized with a standard set of differential varieties. Isolates apparently identical to UA0510C have been found in an increasing number of locations throughout California in 2010 and 2011. After careful evaluation of the significance of this development to the spinach industry, the International Working Group on Peronospora (IWGP) has designated this isolate as race Pfs 13. The IWGP is located in The Netherlands and is administered by Plantum NL.
Race Pfs 13 poses a threat to the spinach industry because it is particularly well-adapted to modern hybrids with resistance to races 1-12. The appearance of a new race is not unexpected because hybrids with resistance to races 1-12 have been widely planted over the past few years. Similar developments have taken place when races Pfs 5 (1996), Pfs 6 (1998), Pfs 7 (1999), Pfs 8 and 10 (2004), Pfs 11 (2009), and Pfs 12 (2009) were identified and named. The occurrence of Pfs 13 will clearly encourage the industry to develop and use new spinach cultivars having resistance to races 1-13. A history of the detection of the various spinach downy mildew races is presented in Table 1.
A collaboration of researchers with the IWGP, University of Arkansas (Correll), and University of California (Koike) is monitoring the development of new races of spinach downy mildew on a global scale by collecting and testing suspected new isolates. In this way it is hoped that research findings and conclusions will be agreed upon and better communicated between the seed industry, spinach growers, and other interested parties. For California and Arizona, the Correll-Koike team will continue to receive and test spinach downy mildew samples for growers, pest control advisors, and seed companies. Industry is encouraged to continue to submit downy mildew outbreak samples to Correll-Koike, as such samples facilitate the discovery of additional new races. The Correll-Koike research is made possible by support from the California Leafy Greens Research Board and by active participation by the agricultural industries in California and Arizona.
For more information on this subject you can contact Steven Koike (firstname.lastname@example.org), Jim Correll (email@example.com), Diederik Smilde (firstname.lastname@example.org), or IWGP chairperson Jan de Visser (JandeVisser@popvriendseeds.nl).
Table 1. Races of spinach downy mildew and year of detection
*One time detection only
Downy mildew of spinach is the most important disease on this crop and results in quality and yield losses.
- 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, Plant Pathology Farm Advisor
The unusually cold, wet, and rainy weather during March through June 2011 has created conditions that favor the development of Botrytis crown rot of lettuce in several counties in coastal California. Botrytis crown rot, also known as gray mold, has resulted in stand loss and reduced yields in numerous fields.
Transplanted lettuce: Crown tissue on transplants in the field becomes brown to orange-brown in color and soft. The characteristic fuzzy gray sporulation of the pathogen is usually present on the affected crown tissue in contact with the soil. Botrytis crown rot causes transplanted lettuce to wilt, collapse, and eventually dry up and die. While still in trays and under greenhouse conditions, lettuce transplants can also become infected with gray mold; however, such infections are usually associated with old, overgrown transplants in which the older leaves have begun to age, turn yellow, and decline (senesce).
Direct seeded lettuce: Direct seeded lettuce can also succumb to Botrytis crown rot. Under most conditions such fields have lower disease incidence and the problem is less serious. However, in spring 2011 a number of direct seeded fields also experienced significant dieback from this disease.
Mature lettuce: Lettuce plants near maturity may appear healthy but develop crown rot close to harvest. Diseased plants will start to wilt, lower leaves turn yellow then brown, and the entire plant will collapse. Examination of the crown will show extensive gray sporulation and the soft, orange-brown decay. Such plants likely were infected earlier in the season and collapsed when the pathogen rotted a significant portion of the lettuce crown.
The pathogen. The causal agent of gray mold on lettuce is Botrytis cinerea. This fungus is a very common organism that readily grows as a saprobe on dead, declining plant tissue and organic matter. The characteristic fuzzy, velvety, grayish brown growth of the fungus can often be readily seen on diseased areas of the lettuce, especially on lettuce crowns in contact with soil and that may be shielded from the sun by overlying leaves. Black sclerotia (hard fungal resting structures measuring from 1/8 to 1/4 inch in diameter) may form on these diseased tissues, although some isolates produce few or no sclerotia. Sclerotia are usually dome-shaped or rounded and may appear similar to sclerotia produced by the species of Sclerotinia (S. sclerotiorum) that produces large sized resting structures. Botrytis cinerea of lettuce is the same pathogen that causes gray mold disease on grape, strawberry, tomato, ornamental plants, and many other crops.
Disease factors: Botrytis crown rot affects all types of lettuce: iceberg, butterhead, green leaf, red leaf, romaine. Botrytis cinerea most readily infects lettuce tissues that are damaged and exposed to moist, wet conditions. For this reason, Botrytis crown rot is most commonly seen on transplanted lettuce. The process of transplanting lettuce results in unavoidable, minor cracks and injuries to the transplant. If B. cinerea inoculum is lacking, such injuries are incidental and cause little concern. However, if the fungus is present then such wounds allow the pathogen to readily invade and colonize the plant crown. Because spring and early summer romaine is often transplanted, most Botrytis crown rot cases in 2011 involved romaine.
Diagnosis: Diagnosing gray mold will require careful examination. Overall plant wilting and collapsing symptoms caused by gray mold may look very similar to such symptoms caused by Sclerotinia minor (lettuce drop) and perhaps Phoma exigua (Phoma basal rot). Accurate diagnosis, therefore, requires careful examination of the crown and perhaps lab confirmation. Note that if rotted lettuce crowns are colonized by white fungal mycelium, the pathogen is likely Sclerotinia.
Control. Because B. cinerea initiates infection on damaged tissues, as much as possible minimize damage to lettuce that is caused by cultural practices, environmental extremes, or other pathogens and pests. Use transplants that are not too large and overly mature; older transplants are subject to additional leaf breakage and damage during planting, and hence are more susceptible to gray mold infection. Limit damage to lettuce transplants during the planting process, though it is not possible to prevent all injury. In the field, reduce leaf wetness by avoiding or reducing sprinkler irrigation. Schedule crop residue incorporation and soil preparation so that excessive plant residues are minimal at planting. It may be helpful to apply fungicides to protect plants from gray mold. However, if weather conditions strongly favor Botrytis crown rot, such applications may be limited in effectiveness. Before using any fungicide for the control of B. cinerea on lettuce, check product labels and your local Agricultural Commissioner's Office for use information and restrictions.
Figure 1. Orange-brown soft rot and gray sporulation on lettuce caused by Botrytis cinerea
Figure 2. Orange-brown soft rot and gray sporulation on lettuce caused by Botrytis cinerea.
Figure 3. Reduced romaine stand due to Botrytis crown rot of transplants.
Figure 4. Lettuce transplant infected by Botrytis cinerea.
- Author: Steven T. Koike
The cucumber crop in central coast California is a minor crop, with only a modest acreage planted annually. However, a major disease threatens this commodity that is grown both out in the field and inside greenhouses. In recent years, a very aggressive, destructive strain of downy mildew (the pathogen is Pseudoperonospora cubensis) has devastated cucumber plantings. Leaves first develop angular shaped lesions that turn yellow. Later, the tissue in these lesions dies and becomes brown (photo 1). In most cases the diagnostic purple gray mycelium and spores develop on the leaf undersides (photo 2). As disease progresses, entire leaves decline and the plants collapse due to severe infection. Downy mildew also infects squash and watermelon, though this current problem is most problematic on cucumber.
California growers are hardly alone in this situation. Last year the aggressive downy mildew damaged cucumber crops in various parts of the USA, along the eastern seaboard stretching from New York down to Florida, and from there extending west as far as Wisconsin, Illinois, Missouri, Louisiana, and Texas. For California, downy mildew was reported on production cucumber in the central coast and other regions, and on seed cucumber crops in the upper San Joaquin Valley.
Management of this apparently new cucumber strain is difficult. Organic producers have few options because protectant sprays do not appear to help, and suitable resistant cultivars have not yet been identified. For conventional growers, early preventative sprays should be made (see the UC IPM website: http://www.ipm.ucdavis.edu/PMG/r116101611.html). This cucumber situation is yet another case illustrating how this group of pathogens is able to change and cause problems for growers. Central coast growers are already very familiar with the new races and aggressive outbreaks of lettuce and spinach downy mildews.
Plant Pathologist Steven Koike is monitoring the California cucumber situation and is collaborating with researchers in Michigan and North Carolina. He is interested in hearing about downy mildew outbreaks on cucumber in California (phone 831-759-7350; email@example.com).
Photo 1: Angular lesions on cucumber caused by downy mildew.
Photo 2: Downy mildew lesions support the purple growth of the pathogen.