Every year farmers face new challenges in producing high quality crops. The 2014 spring cropping season in the Coachella Valley brought quite a few surprises in the way of virus diseases for bell pepper growers.
A basic concept taught in plant pathology classes is the disease triangle which consists of a susceptible host, a virulent pathogen, and a favorable environment. All of the points interact in the field and affect the presence or absence (severity) of disease.
Pesticide applications target the pathogen and vectors. Fertilizer programs aim to keep the host plant as healthy as possible. The environment (e.g. temperature, rain fall, etc.) is an unpredictable factor and may result in plant stresses in the field.
For example, winds can carry insects across the Salton Sea over the low desert areas and into healthy fields. Farmers and pest control advisers (PCA’s) are aware that there is always a potential for environmental conditions to change and favor disease development.
Tomato spotted wilt virus
Early in February, tomato spotted wilt virus (TSMV), vectored by thrips, was found in one bell pepper field in the Mecca area of the Coachella Valley. In this case, the thrips could have arrived with transplants, indicating the need to inspect for insects and disease prior to planting.
Frost protective structures not only protect plants but they can also create a favorable environment for insects. Structures can also limit insect control options.
In March, strong winds damaged bell pepper transplants around the Coachella Valley. Especially hard hit were plantings in the Oasis area. A month later the plants recovered and re-grew normally, and other than a two-week growth delay the damage was no longer visible.
In April, bell pepper fields in the Mecca area showed a bleaching and whitening of the newer growth. These were potential symptoms of nematode damage or nutrient deficiencies.
Sampling for nematodes and plant disease came out negative except for a small number of stubby root nematodes.
Plant tissue samples revealed deficiencies in phosphorus, potassium, and manganese. The problem was remediated when a fertilizer adjustment was made.
Two weeks later, the field had only a few hotspots with the bleached tissue. In this case, tissue sampling was the key to understanding what was occurring in the field. In the meantime, other bell pepper fields began displaying similar symptoms.
Alfalfa mosaic virus
Also in April, the aphid-vectored alfalfa mosaic virus (AMV) appeared in bell pepper fields near the northern border of the Salton Sea. Soon AMV spread out to nearby bell pepper fields. In many crops, including basil, AMV symptoms are severe leaf bleaching.
Cucumber mosaic virus
In May, cucumber mosaic virus (CMV) vectored by aphids was found in fields close to the Salton Sea in the Mecca and Oasis area. In these fields infected with CMV, significant yield losses occurred. Since fields are planted next to each other - separated only by a road separation - it is not uncommon to see CMV hotspots in this area.
Also in May, we received complaints about a red bell pepper field in the Oasis area which allegedly had symptoms of nematode damage. The described symptoms were leaf whitening and bleaching of the newer growth. The symptoms were widespread in the field.
This was troubling to the grower and PCA since bell pepper produces new fruit on younger stem branches. Any chlorosis on this part could limit new fruit development.
The affected plants did not exhibit stunting or leaf deformations that would normally suggest a virus as the cause of the problem. Since virus symptoms can overlap, we collected plant and soil samples for the presence of virus and nematodes and a sub sample was sent to Bob Gilbertson at UC Davis Plant Pathology. Gilbertson found beet curly top virus-svr in the samples.
Root-knot nematode levels in the soil samples were very similar to the numbers counted in previous years from soil around pepper plants displaying similar symptoms: chlorosis, yellowing, and the bleaching of newer growth. What was new was the interaction of beet curly top-svr (BCTV-svr) on these plants.
There were plants in the field that definitely had virus problems, but those plants were not the ones we sampled at the time. Only those plants with younger bleached tissue were sampled. The affected plants had smaller roots and smaller fruit than the healthy plants in the field.
Finding BCTV-svr in these plants added to the confusion since the symptoms were not typical for this virus. On June 12, six new fresh bell pepper samples were collected from the same fields for virus identification.
While three of the samples sent to the UC Pathology laboratory were damaged (rotten) and could not be used for diagnosis, two of the remaining samples were found positive for BCVT-svr and AMV. The symptoms were different from those seen in fields infected with AMV at an earlier stage.
These results linked the two viruses with the chlorosis observed in the field. Due to the poor condition of the samples for disease diagnosis, a new set of six samples was collected and shipped to UC Davis in late June. It was assumed that if the laboratory received good samples the link between these viruses and the symptoms could be better understood.
At this time, the plants arrived at the lab in excellent condition and were tested. Based on the presence of these two viruses, it is clear that this combination of viruses, along with the root-knot nematode infestation, is contributing to the chlorosis, whitening, and bleaching of younger tissue.
Growers and PCA’s scout fields continuously for thrips, aphids and leafhoppers as the pests not only feed on plants but also vector viruses. Weed control in and around the fields is important since weeds can be alternate hosts for the insect vectors and viruses.
Plant tissue sampling is a valuable tool that helps aid in the diagnosis of crop problems. This can help rule out whether the problem is abiotic or biotic in origin. This also provides a grower with information on how his fertility program is working.
Sometimes a crop can be infested with a combination of pests - from nematodes to insect pests and viruses - and the symptoms can be different from what is typical.
A valuable tool in crop disease management is the use of crop rotations and an area that needs more research is the identification of nematode tolerant or resistant bell pepper varieties.
Plant samples must arrive in excellent condition to the laboratory for effective diagnosis. Pay special attention to this, otherwise the results may be misleading or incomplete.
In nematode infested fields proper timing of treatment is important. When harvest is completed, do not to disturb the soil until nematode treatment is concluded. The treatment may not eradicate the nematodes completely but it could reduce the population in the soil.
This article can help others develop effective integrated pest management practices.
We gratefully acknowledge the support of: Bob Gilbertson, UC Davis; Steven Koike and Richard Smith, UCCE Monterey County; and Coachella Valley PCA’s.