Yellow Leaves on a Weeping Willow Tree?

Hi, I am from Houston, Texas - we also have weeping willows.
In the beginning - I did the same - I put my weeping willow ina pot -
the brown spots that you have received are common due to heavy rain - brown spot!
Go to your local nersery - or Home Depot - Lowes - and go to the area where they have the pesticides - they also have remidies there for plants that have contracted Black or Brown Spot - Get a liquid Fungicide - read directions and dilute as the directions say -
If you can MOVE the pot to an area where it is covered for two to 5 days - this will help let the fungicide work and not get any more water!
It will drop the leaves that have been infected - and you can also spray the fungicide on the leaves

Lastly - When the infected leaves drop into the pot - Please remove them - the infected leaves will keep the fungus growing in the soil.
So it is important remove those leaves!
As you see the soil dry - poke your finger in the soil about to your second nuckle - see if it feels moist - wait until you feel this amount of area is dry and wait one more dry - non raining on your plant day and then just sprinkle your plant. Return it to its present position when the weather is dry and sunny. Keep an eye on the dropping leaves to remove old ones and the dryness of the soil and you will start to see new leaf growth about 15 days after the soil feels dryer.
Please plan a garden place for this plant - the rooting system grows quick and will pot bound quickly- good gardening!
Sue - Houston Texas