One plant much smaller than identical siblings

Likely causes, most likely first

Ranked for this sign on its own. Your stage, medium and readings change the order, so run the checker for your plant.

1. PossibleHop latent viroid (dudding)

   DiseaseAct within a day or twoRead: Hop latent viroid (HLVd, 'dudding') and other cannabis viruses

   Fix: No cure in a plant. Test mothers (roots and leaves) by RT-PCR or RT-qPCR, discard positives, disinfect tools with 10% bleach or heat (alcohol and peroxide don't inactivate it), and use meristem tissue culture to rescue valuable genetics. Don't take cuttings from stunted plants.

   Recovery: Negative tests on new mothers; no more duds in the next cycle.

   How to confirm it

   • Lab test for hop latent viroid (RT-PCR or RT-qPCR). Send root and leaf samples (roots detect HLVd earlier in young plants) to a lab offering HLVd RT-PCR or RT-qPCR. Test mother plants and any stunted plant among healthy siblings. A positive plant cannot be cured; tissue culture of meristem tips can rescue genetics.
   • Compare with siblings or the same cut. Put the affected plant beside siblings or clones of the same cut in the same conditions. A trait shared by all of a cultivar (purple stems, foxtailing, pale colour) is genetic; a problem on one plant points to its roots, a virus or its genetics; a problem on every cultivar points to the environment or feed.

2. PossibleWeak or slow individual phenotype

   GeneticsLow urgencyRead: Slow or stunted cannabis growth: a differential

   Fix: Seed-grown siblings vary; a consistently slow plant with healthy roots and a negative HLVd test is a phenotype, not a fault. Don't select it as a keeper or mother.

   Recovery: Healthy roots, negative HLVd test, otherwise normal colour.

   How to confirm it

   • Compare with siblings or the same cut. Put the affected plant beside siblings or clones of the same cut in the same conditions. A trait shared by all of a cultivar (purple stems, foxtailing, pale colour) is genetic; a problem on one plant points to its roots, a virus or its genetics; a problem on every cultivar points to the environment or feed.
   • Lab test for hop latent viroid (RT-PCR or RT-qPCR). Send root and leaf samples (roots detect HLVd earlier in young plants) to a lab offering HLVd RT-PCR or RT-qPCR. Test mother plants and any stunted plant among healthy siblings. A positive plant cannot be cured; tissue culture of meristem tips can rescue genetics.
   • Look at the roots. Hydro: lift the net pot and look at the root mass in good white light. Containers: tip the plant out with a hand over the medium and look at the edges and bottom of the root ball. Healthy roots are white to cream with fine fuzzy tips. Brown roots whose outer layer slides off between the fingers leave a bare thread: rot. Firm, evenly tan roots with white tips in coloured solution are staining.

3. Less likelyRoot aphids

   PestAct within a day or twoRead: Root aphids (rice root aphid) on indoor cannabis: ID, life cycle in numbers and IPM

   Fix: Confirm on the roots. Apply soil-dwelling biologicals (Beauveria bassiana drenches, Stratiolaelaps scimitus, Steinernema or Heterorhabditis nematodes), sanitise, and leave a plant-free gap between cycles; dispose of infested media.

   Recovery: No live aphids on root checks after 2-3 weeks; growth resumes.

   How to confirm it

   • Root-ball and drainage-hole insect check. Tip the plant out and inspect the outer roots, pot bottom and drainage holes with a 10x loupe. Root aphids are small, pear-shaped and wingless, often with white waxy flecks; check several plants, and the drain trays, since colonies hide at the pot edge.
   • Sticky cards for 1-2 weeks. Hang yellow cards at canopy height and lay one flat on the medium surface; add blue cards for thrips. Count catches every 3-4 days. Rising counts of small dark flies at the medium mean fungus gnats; slender insects mean thrips; white wedge-shaped fliers mean whiteflies; winged aphids mean a colony nearby.

4. Less likelyPlant virus (e.g. beet curly top, lettuce chlorosis virus)

   DiseaseDeal with it this weekRead: Hop latent viroid (HLVd, 'dudding') and other cannabis viruses

   Fix: No cure: remove the plant. Control insect vectors (leafhoppers outdoors for curly top; whiteflies for some others) and test clone sources.

   Recovery: No new symptomatic plants over the next 2-4 weeks.

   How to confirm it

   • Send a sample to a plant diagnostic lab. For rots, wilts, leaf spots and viruses you can't separate by eye, send a fresh sample (whole plant with roots or a wilting branch with healthy and diseased tissue) in a sealed bag to a university or commercial plant diagnostic clinic. Results in days to 2 weeks.
   • Magnifier check of leaf undersides and new growth. Use a 10-20x loupe for spider mites, thrips, aphids and whiteflies, and a 30-60x lens or clip-on phone macro for broad and russet mites. Check the undersides of 10 affected leaves across several plants and the newest shoot tips, in white light. Look for moving dots, eggs, shed skins, webbing, slender insects with black specks, and glassy specks on new growth.

Look for

The small plant next to its same-cut siblings..