Nitrogen Toxicity affecting your marijuana plants.
Leaf Color:
Edges Appear Brown or Burnt
Yellow Leaves - Lower, older leaves
Dark or Purple Leaves
Brown or Dark Spots
Leaf Symptoms:
Upper Leaves / Newer Growth Affected
Lower Leaves / Older Growth Affected
All Leaves Seem Affected
Leaf Edges Appear Burnt
Leaf Tips Appear Burnt
Spots
Leaves Curl Under
Wilting / Drooping
Plant Symptoms:
Weak Stems
Leaves Curl Under
Plant Wilting / Drooping
Other Symptoms:
Buds Not Fattening
Fair enough. Come to think of it, the lower leaves that have fallen off, the stems are brittle where it met the stalk. Almost hollow.