Use a fungicide listed for use on begonias and follow label directions exactly. Any balanced fertilizer is fine. Two fungicide … It is wise to keep plants away from cool air drafts and if grown indoors, the temperature should be as even as possible. Subdue MAXX and Segovis are both systemic fungicides and are especially effective when applied as a drench. ... now is a good time to spray with a fungicide. Systemic fungicides can be especially helpful in managing downy mildew because these products are absorbed by the plant in a limited way and can fight the downy mildew pathogen effectively. Powdery mildew, or Oidium begoniae, appears on begonia leaves as a white powder or powdery looking spots. Use the fungicide … Begonias love good air circulation, therefore it is best … It tends to occur most commonly when conditions are more humid in late … This fungus is more prevalent during late summer and fall under humid or dry conditions. Most fungicides are preventative sprays and you must start spraying before you get mildew if you want to control it. Since begonias … Quarter strength fertilizer once a week is the best method. For indoor begonias, choose a fungicide that contains potassium bicarbonate, Bacillus subtilis or neem oil.