Most common symptoms with their most common causes:
No Seedlings—old seeds, cold soil, damping off fungi, eaten by pests
Wilting—too little or too much H2O, leaf-sucking insects, root insects, wilt fungi, nematodes
Mold on Buds or Leaves—grey mold, brown blight, downy mildew, pink rot, powdery mildew (do not confuse mold with webbing)
Webbing—spider mites, budworms, hemp borers, leaf-eating caterpillars
Spots on Leaves—leaf-sucking insects, leaf fungi, leafminers, too much fertilizer
Brown and Curling Leaf Margins—too much fertilizer, dry air, too little potassium, brown blight
Holes in Leaves—caterpillars, beetles, grasshoppers, bacteria
Pale Green or Yellow Leaves—not enough nitrogen, poor pH, nematodes, soil fungi, leaf-sucking insects
Lumpy Stems—European corn borers, hemp borers, beetle grubs, canker fungi, stem nematodes
Spindly Stems—not enough light, too much yellow light, temperature too hot, soil too wet, not enough nitrogen or potassium or calcium
Disfigured Roots—soil fungi, nematodes, broomrape, grubs, maggots, rodents
Tips of Limbs or Tops Missing—rodents, rabbits, deer, cattle or HUMANS
Resource
Hemp Diseases and Pests Management and Biological Control, J. M. McPartland, R. C. Clarke, and D. P. Watson. 2000 copyright