Winter is the prime time for pelargoniums ("geraniums") to be afflicted with botrytis fungus. This fungus is a relative of the mildew that attacks your shower and tub, especially in summer when it is hot and steamy. However, unlike warmth loving mildew, botrytis likes it cool, temperatures in the 50s and 60s range is a prime time for it to show up. If you have your plants in a greenhouse or perhaps a glassed in sunroom, it is most likely that the temperatures will be in the 50 to 60 degree range. If you are a resident of the Pacific Coast area where you are not subjected to the freezing weather that most of the US experiences, your plants will, most likely, still be in the ground, or pots, outside. You may also have problems with botrytis as the weather cools for your winter. Winter rains or fog will make the fungus spread faster as cool, damp, conditions is what this fungus likes. Mildew and botrytis both share their love of dampness.
Like all fungus, botrytis is spread by fine spores, any air movement will disperse these spores, spreading them to other plants in the vicinity. Just the slight air current that you cause by walking by, or in pruning the plants ,will be more than enough to cause the dispersal of these spores. Botrytis feeds best on dead or dying plant material such as dead leaves or stem ends. However, if the fungus has good conditions for it's growth, it will also attack living plant tissue. The dead leaves will not show much, although if the dead leaf you are picking is dry, you might notice a fine dust waft into the air as you pick them This "dust" is the spores being blown about, looking for another home. It will cause the plant flowers to become a gummy, gray mass and black spots may appear on the green stems of your pelly. If damp, cool conditions persist, these initial small black, sooty looking spots, or dots, on the plant stem will start to eat into the stem itself. I have seen stems collapse from this condition.
The best way to fight botrytis is by prevention. There are sprays to use when the disease appears, but this is sometimes not as effective as one might like, especially of the fungus has a tight grip on all your plants. Spraying adds more moisture to the area in which the plants are growing, a condition that the gardener is trying to prevent. Keep all dead leaves, et al, picked off your plants. If the weather, or area where the plants are being grown, is especially damp, thin the branches of your plants, strip leave off in the middle of thick plants, also too many center branches. Do not overcrowd your plants at this critical time. The aim is to get more air circulation into and around the plant to counteract damp conditions. Another plus of thinning the plants, it opens them up so any spray you do use can reach all plant surfaces. The plants may not look great, but at least they will stay alive and will rapidly re-grow to a bushy plant when spring arrives and the weather warms.
If your plants are in a greenhouse, sunroom, or even a cool home window for over-wintering, they will profit from some added ventilation. Air flow aids in keeping the leaves and stems of your plant dry, thus keeping botrytis at bay. The spores may be dormant on the plant leaf, waiting needed moisture for it to multiply. If the plant material is kept dry, the botrytis will not start multiplying its zillion spores. the best way to up the air movement is by using a small fan. In a greenhouse, a fan placed, or hung, above plant top level and aimed at the back of the greenhouse, will keep stagnant air at bay. If the greenhouse is longer than 12 feet, place a second fan at the opposite end so the two fans will create a circular pattern. In the house, a small fan placed to blow just above the plants will help to keep them drier also.
Those who garden in frost free areas will have to be diligent about keeping clean, dead material free, plants, and to keep the centers stripped free of leaves to open them up to what air there is. Pots can be moved to a rain free area, but still must be kept open and airy. Ask your garden center about the proper fungicides for botrytis and use it often if botrytis persists. Rain or heavy fog will allow the fungicide to be washed off the affected plant so be faithful with a spray program. There are some fungicides that state that are residual, they will continue to work even if the rain washes off the initial spray. Some granules for botrytis are sold for putting into the soil for a systemic action. I have tried all products on the market, I have some success with them all, but I still find that the best thing for botrytis is constant vigilance. I have found, that for me, at any rate, the best action is a combination, thinning the plants, no overcrowding, spray if needed.
I do not usually take cuttings from plants that have shown signs of botrytis. If it becomes necessary, use a powdered fungicide and a small paint brush to coat the stem end from which you took the cutting. If not, after the cutting is taken, the plant stem end left will "weep" sap for a short time and will be a great attractant for any fungus spores in the vicinity. They will quickly bore into the stem and can destroy the whole branch and perhaps the plant itself if it is a small one. I have had this happen to me so am always careful about treating the stem end after the cutting is taken.
One last thought, botrytis quickly builds up a tolerance for a fungicide. Buy several different formulations of one and then alternate your fungicide sprays. Be sure and check the label to see if the fungicide you are buying is not just another brand name, but a different formula. Rotate at least two, better three, or more types when you do your weekly spraying. Always remember, although you will not see symptoms of botrytis when it is warm and dry, the spores are, most likely, lurking in wait. The first time cool, damp conditions strike, so will botrytis. Try to keep all plants clean and pruned year round.