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First soybean rust-like spores in state found in SE Virginia
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8/19/2005 10:30 p.m. CDT – A single cluster of six spores matching the description of Asian soybean rust spores was collected in a spore trap in Suffolk, Va., the first sign that Phakopsora pachyrhizi spores may have traveled as far north as the Hampton Roads area in southeast Virginia.
Officials retrieved the cluster from a trap at the Virginia Tech Tidewater Agricultural Research and Extension Center, where the spores had been collected in the air sampling period from August 3 to August 10.
“This is a strong indication that spores of soybean rust have traveled as far north as Virginia,” said David Holshouser, soybean agronomist at the Tidewater center, “and that scouting needs to be intensified until the crop progresses through the most vulnerable stages from beginning pod (R3) up to full seed (R6).
“This does not mean we should begin fungicide applications,” Holshouser emphasized in today’s Virginia state commentary on www.sbrusa.net. He said weather conditions around the time of deposition (Aug. 5-11) were “extremely hot and dry and not conducive for promoting the infection process.”
Conditions for survival, transport and deposition are more favorable with the cooler conditions coming into the state, the commentary said. “The source strength of the latest finds in Georgia and South Carolina are weak. New infections are expected in the next couple of weeks in the South and into the Carolinas. It will likely be two, three or more weeks before rust moves into Virginia.”
Holshouser specifically said there is no indication of soybean rust in the sentinel plots adjacent to where the spores were found or in nearby soybean fields.
“We don't think that fungicide applications to control soybean rust will be needed for most of our full-season soybeans,” he said. “Once the crop reaches the R6 stage, very little yield loss will occur from new infections. On the other hand, our double-crop soybeans are three to four weeks away from the R6 stage.
“We will need to pay close attention to our double-crop soybean fields in case soybean rust is detected in Virginia within the next few weeks.”
In Virginia, sentinel plots (MG 3) are in the R5 (early seed) and R6 (full-seed) development stages. Full-season maturity group 4 soybeans in producer fields are forming seed (R4); group 5 soybeans are forming pods. June-planted double crop soybeans are in the R2 (full flower) to R3 (beginning pod) stages. July-planted double-crop soybeans are only in the R1 (beginning flower) to R2 stages.
Scouting recommendations and techniques:
In light of the spore detections, scouting for the disease needs to be intensified until the crop progresses through the most vulnerable stages from beginning pod (R3) up to full seed (R6). Scout soybeans on at least a weekly basis. Aphids have been detected in Virginia, therefore be on the lookout for that pest as well. If over 250 aphids per plant are occurring and the crop is R5 or younger stage, an insecticide treatment is warranted.
Fields need to be scouted at least once a week, especially from the start of flowering (R1) up to full seed (R6). The intensity of scouting should be increased as the disease moves closer and especially after disease forecasts for movement of windborne spores into Virginia. Scout fields in a zigzag pattern and use different entry and exit points on each visit.
Priority should be given to areas likely to have extended periods of leaf wetness due to poor air drainage as in low-lying areas, and to locations with heavy plant growth and a dense canopy. Also focus scouting in areas of the field that are shaded by the afternoon sun. Attention should be given to checking the leaves that are midway or lower on plants. This is where the disease is most likely to develop first. For aphids, we recommend a whole-plant examination using similar techniques as for soybean rust.
Source: Virginia state commentary 8/19/2005 on www.sbrusa.net.
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