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Crop Diagnostic Lab Report

Week of May 25 - May 28, 2010

The activity level in the CDC is steadily increasing as the growing season is now well underway. We are pleased to have Navreet Deol join us for a 3rd season in the lab but this time as an Associate Diagnostician rather than a student lab assistant.

Cereals Over the past two weeks we have had several samples of wheat affected by wheat streak mosaic virus (WSMV). The story for these is generally the same, winter wheat was on the fields two seasons ago and volunteers were not well controlled. Symptoms of the disease may not be distinctive on seedling spring wheat plants but a yellow mottle of leaves on the infected plants may be noticed with a close inspection. Wheat curl mites vector the disease but are very small and can’t be seen without magnification, and even with a microscope, careful inspection is sometimes required to find them. Early season infection of plants can result in serious crop losses since the infected plants are often sterile. This week’s WSMV was from the Red River GO area and previous samples were from SouthWest and Pembina areas.

Wheat curl mite

Wheat curl mite – these carry the WSMV from season to season on a green bridge of plants such as volunteer winter wheat.

One sample of wheat affected by deep seeding stress and soil crusting problems was received (SouthWest). Deep seeding can result in spindly plants as they struggle a long way to the surface. If any soil crusting is present, they can have a very hard time breaking through. In this case the grower had listed seeding depth as 1.5 inches but we measured more than double that depth on most plants submitted. When the weakened plant is unable to break through the soil crust, the leaves can crumple and fold beneath the surface. The longer the plant remains below the surface the more vulnerable it is to attack by disease organisms.

Wheat deep seeding soil crusting

Wheat – the long white area on the plants to the left of the picture show the stem area from below the soil line. The crinkled yellow plants on the right of the picture were completely below the soil surface and unable to break through the crust resulting in the crinkling of the leaves.

One sample of barley was received that had leaf yellowing (Pembina). This sample showed the leaf yellowing consistently present on the first leaf of all affected plants as well as the tips of the second leaf on some plants and no evidence of root disease was present. When the identical symptoms are found consistently on the same leaves or positions on many plants, an abiotic event that caused some degree of injury, often environmental, is usually involved.

Barley leaf yellowing

Barley – the first leaf of each plant and tips of some second leaves are yellowed indicating that something stressed or injured many plants simultaneously while they were in the same stage of growth.

Forage Legumes One sample of alfalfa this week had leaf spotting (South Interlake) that was found to be due to a combination of three fungi, Stemphylium, Leptosphaerulina and Colletotrichum. One sample of alfalfa had symptoms suggestive of a micronutrient deficiency, showing chlorosis and whitening of leaflet blades, but since only two plants were affected, a root disease was suspected (South Interlake). When nutrient deficiency is present, many plants should be symptomatic, not just one or two. Root rot diseases can impair root function resulting in poor nutrient uptake which may result in deficiency symptoms in some cases.

Fruit One sample of saskatoon was diagnosed with dieback caused by a combination of Cytospora canker and Fusarium solani (South Interlake). Fusarium solani can attack roots or stems of woody plants.

Trees and Shrubs One sample of ash with anthracnose was received (Urban). Brown lesions were present on leaves and since infections had happened early, before leaves had fully expanded, puckering of the leaves resulted when healthy areas of the leaf blade continued to expand. One sample of juniper was received that showed many small dead twigs and tips as well as some larger dead branches (Urban). This sample was found to be affected by Phomopsis tip blight and Lophodermium needle blight. When juniper is affected by a complex of more than one organism, it is often referred to as ‘dieback’. Juniper growing in heavy clay soils or subject to injury from heavy snow cover are often affected by ‘dieback’.

Juniper phomosis & lophodermium

Juniper – many small dead twigs and tips can be seen on this juniper sample affected by Phomopsis and Lophodermium..

Juniper browning twigs

Juniper – the dark football shaped fruiting bodies of Lophodermium can be seen on a dead twig near the top of this picture. They can be seen by the unaided eye as tiny dark dots on dead needles.

Lophodermium fruiting bodies

Lophodermium fruiting bodies - Moisture or high humidity stimulate the fruiting bodies to open by a longitudinal split. Only a few minutes under the hot microscope lights resulted in them closing again.

We had one sample of insect larvae that were reported dropping from trees, type of tree not known (South Parkland). The larvae were very small, about 1 to 1.5 mm long, legless and cream coloured. They were identified as midge larvae. It was unclear whether these were causing damage since none was reported. Exact identification of larvae is difficult. Many members of the midge group are known to be gall formers on trees but depending on the species of midge can also feed on buds, seeds, fungi and an assortment of other things. Many species will drop from trees to complete their life cycle in the soil.

Midge larvae from trees

Insect larvae – these larvae were reported as dropping from trees and were identified as midge larvae.

Weed Identifications Weed identifications for this week were Kentucky bluegrass and slender salt-meadow grass (both Red River).

The Diagnostic Lab Report is prepared by Mardi Desjardins, Crop Diagnostic Centre, Manitoba Agriculture, Food and Rural Initiatives, 201-545 University Crescent, Winnipeg, Manitoba R3T 5S6. Phone: 204-945-7707 Fax: 204-945-4327.

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