Spruce needle rust requires other plants and specific growing conditions to complete its life cycle. In early summer, the rust fungus produces spores on the leaves of Labrador tea or leather leaf, a plant that grows in peat-lands, bogs and swamps. Wind blows the spores onto this year’s needles. They only become infected in wet and cool weather.
All the homeowner of an infected tree can do is water the tree only during the day. Don’t let the sprinkler hit the needles. Water the soil only. Keep grass and weeds down under the tree and mulch to reduce weeds and grass and maintain what moisture is in the soil.
Now that you are thoroughly terrified about yellow needles, there is another reason for yellowing needles, called seasonal needle drop. This is a normal process. Evergreen needles have a finite life. When they die, they turn yellow, then brown and fall off. On a white pine or arborvitae this can be quite dramatic making the tree look like it is dying. If it is a normal needle drop the tree will look uniformly brown in the interior of the tree since that is where the oldest needles are located. In a few weeks, the brown needles will fall off and the tree will look normal and healthy. If you see brown needles in various parts of the tree, not a uniform brown, you may have a sick tree.
Since plant roots are still active, pulling sugars and starches in to store for next summer’s blooms, now is the perfect time to kill perennial weeds. The spray you put on the green leaves will go into the root killing the plant.
Now to fall watering. The temps are cooling off. That doesn’t mean you should put the hose away. Even though plants start to go dormant and perennials start to die back, the root systems are still active and feeding the plant. They need water, especially the conifers. They lose water through their needles all year round. They need to go into winter fully hydrated.
Keep watering the good guys and spraying the thugs until freeze up. If you planted bulbs this fall, water them, too. They start growing a root system now. Any tree or shrub planted less than five years ago needs extra water now, too. Don’t be so tight with water that you lose an expensive tree. There is a difference between thrifty and stupid when it comes to watering plants and trees. Stupid buys a replacement tree nearly every year. Thrifty pays a little extra water bill.