god i hate poisons. sorry for the rant. i think what i meant to say was for your ornamental u can try a soil drench of neem if u take care of the particulars. not neem oil or neem soap tho. neem extract. at proper strengths.
To kill mold organically u can try removing all dead tissues. and then coating everything left in a substance strong in pH. The strong pH change is what kills the fungi. it will have to be applied many times because each treatment can kill a layer. fungi often build in mat-type structures so there can be several layers. and also just because u kill the top of a fungi it does not mean all of it's root reserves are exhausted and so it can grow back. if u kill it over and over it will exhaust it's reserves.
it might be best to use vinegar since the gingko likes acidic soil (5 - 5.5). vinegar is less caustic to (most?) plant tissue than baking soda anyway. a spray at least once every three days is good. 2 teaspoons per quart or litre of water.
i think that the vinegar thing is what i might do. and also i would do the things for the roots that i am about to tell u. and then i would wait because i found this text:
Extreme examples of the Ginkgo's tenacity may be seen in Hiroshima, Japan, where four trees growing between 1–2 km from the 1945 atom bomb explosion were among the few living things in the area to survive the blast (photos & details). While almost all other plants (and animals) in the area were destroyed, the ginkgos, though charred, survived and were soon healthy again. The trees are alive to this day.
While surfing, I saw it said that the gingko wants well draining soil. that (generally) means it will be prone to root rot. A root rot is actually caused by a lack of oxygen. yes it is the roots getting soft and the bacteria eating it that IS the rot but what allows it is the lack of air. because the rot bacteria are airless dwellers (anerobic) and also because air keeps the roots healthy (we can actually grow plants in complete submersion as long as there are enough bubbles in the water).
a cure can be hydrogen peroxide. because it is water with oxygen in it. it's formula is h2o2 and water is h2o so it loses an oxygen and leaves water. the free oxygen oxidizes the bacteria and since they are small they die. the oxygen is also like delicious food to the plant and using h2o2 in regular watering has shown great growth. The strength available through horticultural supply is generally 35% and needs care and dilution because that strength can burn YOU and the plant. The strength you want is 3%, like the kind found for home use at the drugstore.
take it out of the pot and look at the roots. if the soil looks like it has settled into layers, then that may be what caused the excess moisture.
if the roots are brown and fall away easily then that is for sure a bad root rot. it might be best to repot her right away. if the roots u see are not white and wonderful, they still may be functional, so it might be better to wait for repotting until she recovers.
i say to go ahead and repot just because of the rain thing and the possibilty that it's interaction with the soil is what triggered this browning. Unless u specifically mixed that soil with an eye to the plant's indiviual needs, like if u trusted a commercial mix to be good, then there can be something not perfect about it.
that unperfectness may even be true if u get a soil JUST for gingkos. which i would not surprised if u could. because it is very popular for bonsai and those folks make all sorts of specialized mixes.
if the current soil has some peat in it then it is probably too soggy.
I read it wants silty soil. i think that translates to minerals and humic/fulvic materials rather than raw nitrogen or phosphorus.
mineral sand is sold some places, i do not know what is actually in it. kelp has all the micronutrients in it but that may be kind of short acting for a long term pot. it does come in a water soluable form.
minerals come from different colored clays and rocks<
also don't forget the acidity part. Acidity comes from (some?) wood and also iron based clays and i do not know what else. minerals are also found in wood and leaves since the tree can bring them up from the subsoil.
a good wood based soil mix is orchid mix. how many large pieces to sift out depends on ur pot size and the length of time u plan to leave the plant in there.
sand based mixes are cacti and palm. but their sand may just be regular plain silica and not the diversity you may be seeking. u could get lucky and find some designer brand? idk because i haven't really shopped it.
so unless u get better advice, i am thinking what it wants is lots of little rocky stuff/sand and wood pieces with some clay and compost. and then a couple of worms. and then for future care feed the soil from above with raw organic stuff like forest or yard debris/leaves and all.
inside the pot it can be nice to put a layer of pebbles first. it acts as a mesh for more water and air flow.
what kind of water to give it<
a big problem is there is no known wild groves of this tree. so i really got no baseline to go from in my advice. all the advice u get for this tree will be from folks who are telling u their experience and if it is like other cultivation u are going to get folks who have oppositional info to share. but then there will be some parts of the info that will emerge as being constant.
how to grow gingko:
http://search.yahoo.com/search?p=grow+gingko+tree&ei=utf-8&fr=b1ie7
some of those places are, or have, forums where u can ask questions. like the arbor day one will have a text about how to care for ginko but then they will have a forum where u can go to ask further questions. also since they answer questions, u can search their site for ones similar to yours that have already been asked<
for container culture of this plant you are likely to get great advice from the bonsai dudes:
http://search.yahoo.com/search?p=grow+gingko+tree+bonsai+forum&ei=utf-8&fr=b1ie7
for some trees a disease or spraying schedule is a pretty local event. so for that uptodate stuff u can ask ur extension office:
US and territories~
http://www.csrees.usda.gov/Extension/index.html
canada Europe and new Zealand~
http://www.oneglobe.com/agriculture/extnsion.html#euroextension
West Australia:
http://www.agric.wa.gov.au/index.htm
if u are not covered by the above offices then try your closest agricultural university.
