Root Rot

Above-ground symptoms of root rot are often non-specific: stunted growth, yellowing, wilting (especially under heat or moisture stress), and reduced yield or stand loss. Below ground, roots typically appear discolored, ranging from brown to black to reddish-brown depending on the organism involved, and may be water-soaked or soft in oomycete infections. Fine root mass is often reduced, and in some cases the outer root layer sloughs away, leaving a thin central strand.

Pythium and Phytophthora are oomycetes that thrive in saturated or poorly drained soils and produce swimming zoospores attracted to root tissue, making them especially damaging in wet conditions and under frequent irrigation. Rhizoctonia is a true fungus that survives in soil as mycelium or resting structures called sclerotia, does not require free water to infect, and often damages roots and stems near the soil line. Fusarium species survive as durable resting spores called chlamydospores and frequently behave as opportunists on roots already weakened by other stresses.

Because several unrelated organisms produce similar symptoms and often occur together in the same field, or even the same plant, a management approach effective against one causal organism may do little against another. Drainage improvements, for example, primarily address conditions favoring oomycetes, while sanitation and rotation matter more for fungi with long soil persistence.

• Causal organisms differ in the soil moisture and temperature conditions they favor
• Some are oomycetes and some are true fungi, with different control-product classes registered against each group
• The pathogen or pathogens present in a field can shift the priority between drainage, sanitation, and variety choice

Because the causal genera individually have broad host ranges, root rot affects a very wide range of field, vegetable, and horticultural crops. Cereals such as wheat, legumes such as soybean, and vegetables such as tomato are all commonly affected, though the specific organism or organisms responsible vary by crop, soil, and region.

• Wheat: seedling and crown/root rot complexes
• Soybean: seedling blights and root or stem rot complexes
• Tomato: damping-off and root rot, especially in wet or poorly drained soils

• Poorly drained or compacted soils that remain saturated after rain or irrigation
• Cool, wet soils at planting that slow seedling growth and prolong exposure
• Drought stress or heat that weakens roots and favors opportunistic fungi
• Continuous cropping of the same or related host plants
• Injury to roots from nematodes, insects, or cultivation
• Low-vigor seed or transplants

Because root rot can arise from several different organisms, effective management typically starts with correct diagnosis and then addresses the specific conditions favoring the pathogens involved.

• Improve soil drainage and avoid compaction
• Avoid overwatering and allow appropriate drying between irrigations where practical
• Use high-quality, vigorous seed and transplants
• Rotate with non-host or less-susceptible crops
• Choose resistant or tolerant varieties where available
• Scout fields, especially low or wet spots, and consider laboratory diagnosis when symptoms are unclear