Specific Detection of Phytophthora Crown Rot and Anthracnose in Strawberry

2005 REPORT

Specific Detection of Phytophthora Crown Rot and Anthracnose in Strawberry

Frank J. Louws

Dept. Plant Pathology

North CarolinaStateUniversity

RaleighNC27695

919-515-6689

919-515-8795

PRODUCTION RESEARCH

BRIEF INTRODUCTION TO THE PROBLEM:

Strawberry growers face numerous production challenges. High capital costs to establish a strawberry crop make growers keenly aware of the need to adequately manage weeds, diseases, nematodes, insects and mites that prove to be a constant threat. Disease problems are a primary focus of pesticide input and crop losses in strawberry production systems. In particular, anthracnose caused by Colletotrichum species, and Phytophthora crown rot caused primarily by P. cactorum (also causal agent of leather rot in northern production regions), are serious concerns in the industry. In our region, we have experienced severe losses in the last three years in particular, due to either or both of these pathogens and they are typically introduced on contaminated transplants. There is a real need to develop rapid diagnostic assays to allow rapid disease identification and to develop plant quality assays to ensure reduced risk of planting contaminated plants. We have initiated development of standard and real-time PCR assays and sampling protocols to aid in our goal to ensure the use of disease free plants.

In 2005 we articulated the following objectives: 1) to evaluate the potential of specific detection and diagnosis of Colletotrichum acutatum, C. gloeosporioides and Phytophthora cactorum to monitor production of “disease-free” transplants; 2) to develop a real-time PCR assay to rapidly identify the presence of Colletotrichum (with emphasis on the acutatum strain) or Phytophthora in diseased plant tissue and 3) to initiate work to develop sampling protocols to detect latent infections of these pathogens in nursery production systems.

During 2005, we did not focus on our Phytophthora work. The Phytophthora work presented here was completed during phase 1 of this project. We have done extensive work with Colletotrichum assays in 2005.

DESCRIPTION OF THE EXPERIMENTAL DESIGN:

Phytophthora Work (2004):

Identifying Phytophthora cactorum morphologically requires familiarity with the shapes and sizes of oomycete sexual structures. It can take several days and may require multiple steps to encourage cactorums to reproduce sexually. Even under good conditions some specimens of cactorum may not form these structures and could require molecular identification. To make PCR based identification faster and cheaper, we have combined four primers (multiplex-PCR); CACTF1, CACTR1, DC6 and ITS2 (Fig. 1). Together, they form four bands for “cactorums”, one band for other oomycetes and no band for all other organisms. Eddie Croom (undergraduate research assistant); L.M. Ferguson; L. Leandro and F.J. Louws. We have used this multiplex assay extensively to identify and characterize problems associated with strawberry root and crown rot problems. An hourly student can competently assay 40 samples in an afternoon.

ladder 1 2 3 4 5 6 7 8 9 10 ladder Lane #

Figure 1:The ladders are the 100bp size standards on either end. For lanes 1 and 8 the samples do not contain any oomycete DNA. They have no bands. In lane 3, the sample is cactorum and has four bands. Three of these bands are unique for cactorum. All other lanes have the one band that is formed from oomycete DNA.

Colletotrichum Work:

In phase 1 of the project (2004) we compared DNA extraction kits to determine the best kit to obtain fungal DNA that could be assayed by PCR. Of all the kits, only the MoBio PowerSoil kit yielded consistent bands. We successfully adopted or adapted primer sets from the literature and could reliably amplify C. acutatum, C. gloeosporioides and C. fragariae (the later two could not be differentiated by PCR).

IN 2005, a real-time PCR protocol was initiated using the TaqMan technology. Specific primers were developed for Colletotrichum (based on “acutatum” sequences although we anticipate, but have not proven, that the primers will amplify DNA from all three species) as well as a specific probe. The sensitivity of the PCR assay was determined using purified DNA as well as DNA spiked into strawberry petiole extract (to determine if strawberry samples will inhibit or affect the results). Data on these experiments is being compiled.

The assay was then evaluated on inoculated plants in controlled experiments in growth chambers as well as on field samples during a Colletotrichum epidemic in several production nurseries.

Strawberry plants were obtained from the NCSU Micropropagation Unit and grown in the NCSU Phytotron growth chambers. Two sets of 48 plants were sampled before inoculation and 5, 12, and 19 days after inoculation. The infected petioles had an average detection rate of 4.3% by the Paraquat assay (standard method of detecting latent infections of the pathogen) and 79.9% by the realtime PCR assay. The uninfected petioles, however, showed a 0% detection rate by the Paraquat and 4% by the realtime assay in the first run of the experiment. A second run is currently under analysis. The realtime assay was also tested within a field setting where there was a natural epidemic of C. gloeosporioides; 60 petioles were collected from each of three cultivars in the field and tested by the Paraquat assay. Thirty petioles of each were also tested by the realtime assay. The Paraquat and realtime assays yielded 6.6% and 0% for ‘Bish,’ 10% and 13% for ‘Sweet Charlie,’ and 100% and 70% for ‘Chandler,’ respectively. In the ‘Bish’ assay, 25/30 petioles showed late amplification, indicating that there were fewer than 10 DNA copies per reaction. The lower detection rate seen in the ‘Chandler’ petioles may be due to high levels of wound phenolics, which inhibit PCR, suggesting additional work is needed to manage inhibitors in the assays. Detection of the pathogen in planting stock by realtime-PCR has the potential to become an important technique for management of strawberry anthracnose.

SUMMARY OF RESULTS AND CONCLUSIONS:

Primers have been adapted or developed for the specific detection of Phytophthora cactorum and Colletotrichum species. We have used these protocols for rapid diagnosis and identification of these serious pathogens in commercial samples and controlled experiments. A real-time PCR protocol has also been developed and further work is underway. We plan to adapt these protocols for rapid detection and identification of plant material. We have met some challenges in extracting pathogen DNA from some of the tissues in attempts to develop protocols that would be useful in “clean plant” programs. Work will continue. Good progress has been achieved. We have utilized $5000.00 of NASGA research funds to accomplish an estimated $25000 to $30000 worth of research (in direct costs) in 2005.

Evaluation of strobilurin fungicides (Abound and Cabrio),

Potassium phosphite and Ridomil Gold for control of strawberry leather rot and Vascular collapse, caused by Phytophthora cactorum.

Michael A. Ellis and Angel Rebollar-Alviter

Department of Plant Pathology, The Ohio State University/OARDC

1680 Madison Avenue, Wooster, OH44691

Phone: 330-263-3849, Fax: 330-263-3841, e-mail:

Leather Rot

Studies were conducted to determine the post-infection or curative activity of Abound, Cabrio, Potassium Phosphite and Ridomil Gold EC for control leather rot. Commercial formulations of mefenoxam as Ridomil Gold 2E, potassium phosphite as Agri-Fos, azoxystrobin as Abound, and pyraclostrobin as Cabrio were used in greenhouse experiments. The concentration of each fungicide used in the experiments was the label recommended rate in 50 gallons of water/A. To evaluate post-infection (curative) activity of the fungicides, inoculated plants with fruits were incubated in the dark in a mist chamber for 12 hours at room temperature to insure infection. After this time, plants with fruits were sprayed to runoff with each fungicide at 12, 24, 36 and 48 hours after inoculation. Pre-infection (protectant) activity was evaluated by applying the fungicides at 2, 4 and 7 days before inoculation. Diseased and healthy fruits were recorded 5 days after inoculation.

Results and Conclusions:

1.)All fungicides provided excellent protectant activity for at least 7 days with, no significant differences between fungicides.

2.)Significant differences in leather rot control were observed when the fungicides were applied after inoculation.

3.)The systemic fungicides mefenoxam and potassium phosphite provided excellent post-infection activity for up to 36 h. Mefenoxam provided a moderate level of disease control at 48 hours after inoculation.

4.)Strobilurin fungicides did not provide satisfactory curative activity. Although a

certain level of control was observed at 12 hours after inoculation, no control was observed when these fungicides were applied after this time.

Based on the results from greenhouse studies, a field trial was conducted to evaluate the post-infection activity of the previously mentioned fungicides under commercial production conditions. Fungicides were applied on a calendar based program on a 7-day schedule or as curative treatments following flooding events. Fungicides were applied on the calendar based program on the following dates: May 19 ( late bloom , green fruit present ) ; May 26 ( fruit set, no bloom and green fruit present ); June 6 ( fruit starting to ripen ) and June 10 ( mature fruit present, first harvest ). AGRI FOS was applied alone on all dates of the 7-day program in treatment one. In treatment three, Abound was applied on the first two dates and AGRI FOS was applied on the second two dates in an alternating program.

Ridomil Gold and AGRI FOS were applied alone as curative treatments after flooding events. Ridomil Gold was applied on May 22 and June 12 in response to flooding events (infection periods) on May 20 and June 10. AGRI FOS was applied on May 22, June 5, and June12 in response to flooding events on May 20, June 3, and June 10. All ripe and diseased fruit were harvested from each plot on 10, 14 and 17 June. The total number of leather rot-infected and marketable fruit, and total yield (weight) were recorded for each replication and harvest date.

Results of 2005 field trial:

Removing straw from between the rows and repeatedly flooding test plots resulted in a mean disease incidence of 66% for all three harvest dates (table1). All fungicide treatments had significantly less leather rot than the untreated control, and there were no significant differences in leather rot incidence between any fungicide treatment. Ridomil Gold and AGRI FOS appeared to provide good control of leather rot when applied as curative treatments. Ridomil and AGRI FOS in curative treatments provided leather rot control equal to that of the calendar based treatments with two and one fewer applications, respectively.

Anthracnose fruit rot developed in the trial, and some interesting results were observed. The untreated control had lower levels of anthracnose than Ridomil Gold. We believe this is due to the fact that the control had very high levels of leather rot infected fruit, and fruits infected with leather rot could not be infected by anthracnose. Ridomil Gold had a significantly higher incidence of anthracnose than most other treatments. We believe this occurred because Ridomil Gold provided excellent control of leather rot, but has no efficacy against anthracnose. This suggests that Ridomil may need to be applied in combination with other fungicides that have activity against anthracnose during wet years when both diseases are present. The alternating program of Abound and AGRI FOS provided excellent control of both leather rot and anthracnose. Our data also suggest that AGRI FOS ( potassium phosphite ) may have some activity against anthracnose fruit rot.

Results and Conclusions:

1.)Mefenoxam (Ridomil Gold) and AGRI FOS ( potassium phosphite) both provided good to excellent control of leather rot when applied in a post-infection ,curative program.

2.)Both of these fungicides have potential for use in a disease forecasting system for

leather rot.

3.)Azoxystrobin (Abound) in an alternating 7-day calendar based program with potassium phosphite (AGRI FOS) provided excellent control of both leather rot and anthracnose fruit rot. Potassium phosphite applied on a 7-day protectant program provide good control of leather rot and a fair level of control of anthracnose fruit rot.

Crown rot or Vascular Collapse

We have developed a technique for inoculating strawberry crowns and roots with Phytophthora catorum that results in the reliable development of crown rot symptoms. We are currently conducting studies to determine the efficacy of these fungicides for control of Phytophthora crown rot.

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