Dr. Vikram Bisht
Potato & Horticultural Crops, Manitoba Agriculture, Foods and Rural Development
Late blight has been confirmed in northern Adams County, Wisconsin (Amanda Gevens, WI). The DSVs had reached >18 a few weeks ago. The strain identification is not known yet.
There is no late blight report in Manitoba. Here in Manitoba DSVs are still much below the critical value of 18, though some areas are nearing the threshold value. The crops are growing well with good moisture and heat in most of Manitoba. Irrigation pivots have been going on in some areas. Many crops are in hooking to pea sized tubers; some fields have 1" tubers. In most fields the crop is closing in between rows. It is important to have the foliage within canopy protected for late blight before complete close-in between rows. Depending how long the foliage remains wet in certain fields depending upon topography and shelterbelt trees, some growers may consider application of locally systemic fungicides.
P-Days accumulation has almost reached the early blight threshold of 300 in most areas. It is recommended to scout for early blight in fields and consider early blight management especially for early maturing varieties. Protectant fungicides provide good protection at low severity of early infections.
Some fields are showing up purpling and curling of top leaves, sometimes caused by Rhizoctonia infection. However, these symptoms were caused by blackleg bacterial infection. A few virus infected plants were observed in a few fields, but the percentage is very low.
In some fields herbicide injury was observed in commercial non-seed crops. Glyphosate drift or overspray from neighbouring RoundUp Ready crops, and also delayed application after a few plants emerged caused these injuries. The injury to tuber production may be minimal, but the risk of injury to seed crop could be significant. There was a report of group 2 herbicide damage also. A few fields had reported Group 4 like injury on young leaves, but as the crop grew, the symptoms disappeared.
RoundUp over spray from RR soybean field Photo: Vikram Bisht, MAFRD