An early start to the growing season, combined with "relatively dry conditions" over much of southwestern Ontario, have given a quick start to development of soybean cyst nematode (SCN), the provincial ag ministry warns.
Over the past two weeks, the pearl white to yellow female "cysts" have been easily found on young soybean plants, according to field crop plant pathologist Alberta Tenuta of the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA) at Ridgetown.
Nematode feeding damages the root system and prevents or inhibits the uptake of water and nutrients, Tenuta wrote in a provincial crop pest newsletter Tuesday.
"In many cases, SCN symptoms may not be obvious in a field until populations build significantly and may go unnoticed for many yields," Tenuta wrote. "For this reason it is important to examine roots for SCN cysts."
SCN symptoms below the ground will include dwarfed, stunted and discoloured roots with fewer nitrogen-fixing nodules. The most obvious sign of SCN infection, however, is the presence of the adult female "cysts" on the roots — white to yellow-brown cysts less than one millimetre in diameter, about the size of the period at the end of this sentence.
Don't just pull up a plant to check for SCN on the roots, the province warned. If you do so, too much root will be lost and the nematodes will be stripped off. You'll want to use a shovel and dig up the plant along with the soil surrounding the roots.
SCN symptoms above the ground, meanwhile, include in many cases no yellowing of the leaves, stunting of plants, and early maturity, particularly on lighter soils under dry conditions or stressed areas in the field.
Damage often occurs in circles and is often confused or misdiagnosed as nutrient deficiency, flooding, herbicide injury, compaction, drought or root rot damage, Tenuta warned.
Yellowing of the leaf margins can resemble potassium deficiency symptoms. Adding potassium will not reduce the damage from SCN or eliminate symptoms, the province said.
Nematode injury symptoms -- including plant death -- are most obvious under stressed growing conditions, especially hot, dry environments, the province said. Under good growing conditions, the visual damage from soybean cyst nematode may go unnoticed.
Under high-stress conditions, however, even low SCN numbers cause considerable visual damage and high yield loss. SCN infection symptoms may not be obvious and yield reductions of 25-30 per cent on susceptible fields can occur without showing visible symptoms above the ground.
Areas of the field where above-ground SCN symptoms will most often occur include entrance points for equipment into the field, equipment and vehicle storage areas, tops of knolls, compacted headlands and along the fencerow where windblown soil tends to accumulate.