Tomato growers often find black stems a major headache—but don’t worry. The causes vary depending on where the blackening occurs, and so do the treatments.
1. Blackening at the Stem Base (near the roots)
Collar Rot: Caused by Rhizoctonia solani. Common at the early planting stage. Symptoms include sunken brown lesions at the stem base, later covered with light brown web-like mold. Roots may look normal, but plants gradually wilt and die.
Control: Avoid deep soil coverage at planting. Drench with hymexazol + metalaxyl when symptoms appear.
Phytophthora Root Rot: Caused by Phytophthora parasitica. The stem base turns dark brown and rots, with discolored vascular bundles inside.
Control: At early onset, drench with mefenoxam + fludioxonil. Reduce watering and avoid waterlogging.
2. Blackening on Upper or Middle Stems
Late Blight: Caused by Phytophthora infestans. Water-soaked black lesions form on stems, expand into cloud-like sunken patches, and may develop white mold under high humidity.
Control: Spray cymoxanil + azoxystrobin, focusing on stems and the underside of leaves.
Stem Canker: Caused by Alternaria spp. Characterized by oval brown cankers that extend into vascular tissue, eventually causing wilting.
Control: Spray prochloraz + prochloraz-manganese, and remove infected leaves.
Tomato Spotted Wilt Virus (TSWV): Attacks stems near growing points. Dark brown streaks appear on the surface but do not penetrate vascular tissue.
Control: Spray moroxydine hydrochloride + zinc fertilizer, and control aphids (the main vector).
3. Prevention Tips
Treat soil with carbendazim before planting to disinfect, and avoid continuous cropping.
Ventilate after irrigation to reduce humidity inside greenhouses.
Remove and destroy diseased plants promptly; spray surrounding plants preventively.
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