saunf and dhania plants getting swollen lumps on stems and umbels, seed yield poor
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Those swollen lumps are stem gall, a fungal disease (Protomyces) that hits coriander and fennel and badly cuts seed yield. It spreads through infected seed and crop residue and is worse in cool humid weather. Use disease free certified seed and treat seed before sowing, since seed borne infection is the main carrier. Remove and destroy galled plants and old residue, avoid dense sowing, and rotate the field rather than repeating these crops on the same land. Resistant or tolerant coriander varieties exist and are worth asking for. For seed treatment and any spray, dose and interval, follow your KVK recommendation and the product label.
Switching to certified clean seed cut the galls a lot for me. When I saved my own infected seed it kept coming back every year. Clean seed is the cheapest fix.
I pull out and burn the galled plants the moment I see the lumps, do not leave the residue in the field. Leaving the old stalks lying around carried the disease into the next crop for me.
Ask the KVK for a tolerant coriander variety, that helped reduce my galls along with clean seed. Also do not sow dhania or saunf on the same plot back to back, rotating the field gave the disease less to live on.
Do not sow too thick, the dense crowded patches galled the worst in my field where the air stayed damp. A bit wider spacing let the plants dry off and the galls were fewer there. Crowding plus the cool humid spell is what set it off for me.
Treat the seed before sowing with what the KVK advises, that with clean certified seed broke the cycle for me. The disease was riding in on my own saved seed every year. Once I stopped saving infected seed the galls dropped off.
After a bad gall year I gave the plot a break from dhania and saunf for two seasons and put something else there. When I came back the galls were much less. The fungus lives on in the old residue and soil, so a rotation break really helps.