Germinated 6 northern lights auto in rockwool with filtered water in the hydro system, no nutrients. After plants reached about 3 inches with small leaves three of the plants had the stems break near the tops and fall over. they looked consistent with pictures of plants with damp out virus. The remaining plants are alive but not looking very good. Any ideas as to what caused this, what to do to save remaining plants. Could the virus have been on the seed when I received them?