Progress 04/01/02 to 09/30/05
Outputs
The phytoplasma disease of watercress that has been affecting farms on Oahu since 2001 is under control. The incidence of the disease has decreased markedly over the last year, due to continued efforts by farmers to eliminate infected symptomatic plants, and to control populations of the insect vector, Macrosteles sp., in their fields. The watercress farmers on Oahu are aware of the problem this disease presents and of the most efficient ways to protect their farms from this threat. Further characterization of the phytoplasma infecting watercress in Hawaii has included the cloning and sequencing of the Rp and Tuf genes in addition to the cloning and sequencing of the 16S/SR/23S ribosomal DNA genes previously characterized. Sequence analysis of the Rp and Tuf genes has been used by many workers to further classify phytoplasmas of the Aster Yellows group. Our analyses of these genes has revealed that the watercress phytoplasma is probably most closely related to the
Aster Yellows-type phytoplasma that causes Onion Yellows in Japan, and is less closely related to the phytoplasma that causes Severe Aster Yellows in western North America. We have completed work on the host range of the insect vector, Macrosteles sp., testing the suitability of selected vegetable and native Hawaiian plants as food sources and oviposition hosts for the insect. We are still conducting experiments using laboratory-reared Macrosteles sp. to transmit the phytoplasma to various hosts under controlled conditions in the laboratory, and to examine the effects of the phytoplasma on this insect vector.
Impacts
We have identified the phytoplasma as an Aster Yellows strain most closely related to the Onion Yellows phytoplasma in Asia, but it is also closely related to the phytoplasma that causes Severe Aster Yellows in western North America. Our work has enabled the watercress farmers of Oahu to successfully manage the yellows disease of watercress caused by a phytoplasma newly introduced into the state. The incidence of yellows disease caused by this phytoplasma on watercress farms on Oahu has decreased markedly over the last year, due to continued efforts by farmers to rogue out infected, symptomatic plants, and to control the population of the insect vector, Macrosteles sp., in their fields. Our work on this disease, identifying the phytoplasma that causes it, and determining some of the transmission characteristics of the insect that is the vector of the phytoplasma has allowed us to make recommendations for disease control that have enabled watercress farmers to regain
most of the production that was lost in the early stages of the watercress yellows epidemic on Oahu, and has prevented the spread of this pathogen to other islands in Hawaii.
Publications
- No publications reported this period
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Progress 10/01/03 to 09/30/04
Outputs
The incidence of yellows disease caused by phytoplasma on watercress farms on Oahu has decreased markedly over the last year, due to continued efforts by farmers to rogue out infected, symptomatic plants, and to control the population of the insect vector, Macrosteles sp., in their fields. All of the watercress farmers on Oahu are now aware of the problem this disease presents and of the most efficient ways to protect their farms from this threat. Further characterization of the phytoplasma infecting watercress in Hawaii has included the cloning and sequencing of the Rp and Tuf genes in addition to the cloning and sequencing of the 16S/SR/23S ribosomal DNA genes previously characterized. Sequence analysis of both the Rp and Tuf genes has been used to further classify phytoplasmas of the Aster Yellows group throughout the world. Our analyses of these genes has revealed that the watercress phytoplasma may be most closely related to the phytoplasma that causes Onion
Yellows in Japan, and is probably less closely related to the phytoplasma that causes Aster Yellows in western North America.
Impacts
Our work has directly helped the watercress farmers of Oahu control the outbreak of a yellows disease in watercress caused by a phytoplasma newly introduced into the state. Our work on this disease, identifying the phytoplasma that causes it, and determining some of the transmission characteristics of the insect that is the vector of the phytoplasma has allowed us to make recommendations for disease control that have enabled watercress farmers to regain most of the production that was lost in the early stages of the watercress yellows epidemic on Oahu, and has prevented the spread of this pathogen to other islands in Hawaii. All of the watercress farmers on Oahu are now aware of the problem this disease presents and of the most efficient ways to protect their farms from this threat.
Publications
- No publications reported this period
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Progress 10/01/02 to 09/30/03
Outputs
We have used polymerase chain reaction assays (PCR) to identify a pathogen that is consistently associated with watercress yellows on all the watercress farms on Oahu. Sequence analysis of the cloned PCR products has confirmed the identity of the phytoplasma as the severe strain of Western North American Aster Yellows (SAY). We have not found any other pathogens associated with this disease of watercress. We have conducted surveys of the incidence of watercress yellows every two weeks for the past six months on the largest watercress farm on Oahu. During the period from May 1 to July 1, 2003, the incidence of SAY on this farm increased to levels as high as 50-75% in some plots. During this time period, populations of the most-likely vector, Macrosteles sp. nr. severini, a leafhopper species, were very high on this farm, which contributed to the rapid spread of this disease within this farm. We have initiated the rearing of the leafhopper that is most closely
associated with the transmission of this pathogen within the watercress farms. We now have established colonies of this vector on plant hosts other than watercress on which they are able to complete their lifecycle, and have begun using these colonies to conduct controlled transmission tests of the pathogen to different plant species. The insect colonies that we have established will enable us to address specific questions regarding the details of pathogen transmission to various crops by this insect. The identification of the causal agent of watercress yellows has been the major step in devising control strategies to limit the spread of this disease on watercress farms in Hawaii. The recommendations that we have provided watercress farmers have allowed them to begin to reclaim the production levels that they were achieving before the introduction of this pathogen and its insect vector into the state. All the watercress growers on Oahu now have a clearer picture of the threats imposed
on their operations by SAY and its vector M. sp. nr. Severini, and through our efforts, have the means to control the spread of this pathogen on their farms.
Impacts
As a result of our work, the watercress farmers on Oahu are now aware of the pathogen that causes the yellows disease of watercress and the importance of controlling both the incidence of disease in their crop and the leafhopper that is the vector of the pathogen that causes this disease. The information that we have provided to the watercress farmers, and the recommendations for controlling the disease that we have made to them have given watercress farmers the ability to control the incidence of SAY in their crop.
Publications
- No publications reported this period
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Progress 10/01/01 to 09/30/02
Outputs
Watercress (Nasturtium microphyllum Boen. ex Rcbh.) in Hawaii has recently been afflicted with a yellows disease with symptoms that include reduced leaf size and leaf yellowing, and occasionally witches brooms. These symptoms are followed by the rapid collapse of affected plants. Watercress yellows disease has led to an 80-90% loss for one of the largest watercress farms on Oahu. The disease is spreading to other watercress farms in the area, and poses a threat to watercress production statewide. Watercress plants that displayed either early yellowing or advanced symptoms of disease were analyzed with phytoplasma-specific PCR primer-pairs P1/Tint and P1/P7. Normal, healthy-looking watercress plants were negative in these tests, but all samples that had symptoms of this disease produced amplicons of the expected sizes using each primer pair. These tests were repeated and the association with phytoplasmas was confirmed with additional watercress samples from the
original site and other nearby farms. Several weed species collected from the vicinity of the most seriously affected farm also tested positive for phytoplasma infection. We have cloned and partially sequenced the PCR amplicons generated from these tests; sequence analyses of these clones indicate that this phytoplasma is very similar if not identical to the severe strain of the western aster yellows phytoplasma (SAY: 16SrI-B group).
Impacts
This phytoplasma affects many crops in the western United States where it has been established for many years. The known host range of the aster yellows phytoplasma is very large and includes monocot and dicot species from at least 40 plant families, many of which are important components of diversified agriculture in Hawaii. This phytoplasma has the potential to significantly impact the production of many vegetable and floral crops in Hawaii, and also may be a threat to many of the plants that are found in our unique ecosystem.
Publications
- Borth, W.B., Hamasaki, R.T., Ogata, D., Fukuda, S.K. and Hu, J.S. 2002. First report of phytoplasmas infecting watercress in Hawaii. Plant Dis. (In press)
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