TheNaturePlanet

Is it a headache when tomato stems turn black? Don't panic! Today, we'll guide you step-by-step to identify the causes and apply targeted solutions!


### I. Black stems at the base: Check roots and vascular bundles first
#### Stem base rot
- **Causative agent**: *Rhizoctonia solani*! Most common during early transplanting. Brown sunken spots encircle the stem base, later developing light brown cobweb-like mycelium. Roots remain intact, but the plant wilts and dies gradually.
- **Solution**: Drench roots with hymexazol + metalaxyl. Avoid excessive soil covering during transplanting.

#### Phytophthora root rot
- **Causative agent**: *Phytophthora parasitica*! The stem base rots dark brown, and brown vascular discoloration is visible when剖开 (split open). The key difference from stem base rot is whether the vascular bundles are discolored—make sure to identify this!
- **Solution**: In the early stage, drench roots with metalaxyl-M + fludioxonil. Control watering to prevent waterlogging.


### II. Black stems in the middle-upper part or branching areas: Check symptoms and location
#### Late blight
- **Causative agent**: Phytophthora. Water-soaked black-brown spots appear on stems, expanding into cloud-like sunken rings. White mycelium forms in high humidity.
- **Solution**: Spray with dimethomorph + azoxystrobin, focusing on stems and the undersides of leaves.

#### Stem blight
- **Causative agent**: *Alternaria* spp. Elliptical brown ulcer spots form, invading vascular bundles and eventually wilting the plant.
- **Solution**: After infection, spray with difenoconazole + prochloraz. Remove diseased leaves to reduce spread.

#### Streak virus disease
- **Causative agent**: Viruses attack stems near the growing point, causing epidermal black-brown spots without vascular bundle invasion. Virus inhibitors are essential.
- **Solution**: Spray with moroxydine hydrochloride + zinc fertilizer promptly, while controlling aphids (the main virus vectors).


### III. Key preventive measures for daily care:
- Disinfect soil with carbendazim before transplanting; avoid continuous cropping.
- Ventilate promptly after watering to reduce greenhouse humidity (high humidity fosters diseases!).
- Remove isolated diseased plants immediately and spray surrounding plants for prevention.