Manitoba Insect & Disease Update - July 11, 2018

Diseases: Goss's wilt was observed near St. Claude.

Early symptoms of Goss's Wilt were observed near St. Claude last week. Symptoms of the disease often move in following wounding from hail and/or sandblasting. This disease has been found in almost all corn-growing regions of Manitoba and may cause yield loss depending on the size of the affected areas in the field and the stage when the crops develop symptoms. Typical symptoms include tan coloured leaf lesions, especially on plants along field edges. The edges of these lesions may appear water-soaked and freckling within the lesion (especially along the margins) will be evident. Under humid conditions, the bacterium causing the disease releases an exudate that will appear shiny in direct sunlight. Since this disease is caused by bacteria fungicides are not effective in its management.

Insects: Grasshopper populations are being monitored in and around the edges of some fields. Armyworms have been noticed in some cereal fields in the Central and Eastern regions. No soybean aphids have been reported yet.

Armyworms are present in noticeable levels in many areas of Central Manitoba. Larvae can be scouted for by counting the number of larvae per square foot on average. The majority of larvae will be on the ground during the day and often hidden under debris on the soil, under clumps of soil or in crack in soil, so scout the ground thoroughly. Many will curl up when disturbed. The colour of the larvae can sometimes vary considerably between individuals. Cereal crops can tolerate considerable defoliation from armyworm, and the threshold suggested in cereal crops is 4 or more armyworms per square foot on average. Populations can be very patchy in a field.