Soybean health is compromised by several plant pathogenic fungi that cause seed rot, seedling mortality, root and stem decay, or premature decline of stems and foliage. Although infection may occur early, many of these pathogens do not cause apparent symptoms until later growth stages.
Frequently, seedling health is ignored because plant populations are acceptable and stem and leaves do not express symptoms during early vegetative growth. Phytophthora sojae, several species of Pythium fungi, and Rhizoctonia solani are thought to be the most important seedling pathogens, but other plant pathogens are actively invading plants from growth stages VE to V4 as well. Infection may cause chronic symptoms, or it can remain latent and cause symptoms of plant decline during the reproductive growth stages.
Plant health assessment is important during the seedling phase. This information can be used to make adjustments in crop management in subsequent years, and may explain symptoms later in the season and less-than-anticipated yield at harvest. Be aware that symptoms caused by pathogens can be confounded by symptoms caused by herbicides and other abiotic causes of plant stress.
Common soybean seedling infections
||Cause (fungus)||Growth Stage||Symptoms||Control||Comments|
|V0-VE||Soft decay of seed; missing seedlings in row.||Fungicide treated seed, Phytophthora– resistant variety.||Favored by cool and wet soils. Phomopsis comes with seed.|
|VE-V4||Yellow, wilting leaves, followed by necrosis; leaves remain attached to stem.||Fungicide treated seed, Phytophthora- resistant variety.||Phytophthora is most common cause of early seedling mortality in Wisconsin.|
|Root and lower stem decay||Rhizoctonia
|VE-V6||Reddish-brown lesions on taproot and hypocotyl; usually superficial; Phytophthora causes brown lesions on stem above soil-line.||Fungicide treated seed, Phytophthora resistant variety; ridging soil around stems by cultivation simulates new roots.||Except for Phytophthora, above ground plant parts may not express symptoms.|
|Premature decline of foliage and stems||Rhizoctonia
Syndrome – SDS),Phialophora
(Brown Stem Rot – BSR)
but infection occurs much earlier
|Wilt, chlorosis and eventually necrosis of leaves; inter-veinal tissues progress from yellow to brown, but major veins remain green (SDS & BSR); internal browning of stems (BSR).||Fungicide treated seed; variety selection||Brown stem rot (Phialophora) & sudden death syndrome (Fusarium) cause unique symptom patterns on leaves; general decline may be due to Rhizoctonia or Mycolepto-discus|