Bonsai and infections and pests
In March of 2005 we had the pleasure of Dr. Robbie joining us from U.C. Berkeley. Dr. Robbie is a well respected plant disease specialist and he shared his passion for all the things that plague our trees. Note that Dr. Robbie and the U.C. Berkeley staff have a plant clinic on the 1st Saturday of every month from 9am to 12pm in the Berkeley Arboretum. Please bring samples in zip-lock bags.
The trouble we encounter with our trees can be split up in to two types: those caused by the environment (these are non-infectious) and those caused by organisms. Examples of the former include sunburn, drought, nutrient deficiencies, and toxins. Examples of the latter include funguses, insects, viruses, and bacteria.
In general, the non-infectious problems are related to how we care for our plants. By paying attention to the trees we can catch these early (usually) and unless there is death in the tissue of the plant, fixing the environmental issue will solve the problem.
Sodium damage, for example, is often caused by over fertilization. The damage shows up in the older leaves first and the tissue on the leaves dies around the edges of the leaf, all the way around the entire leaf, working its way in. You can prevent sodium damage by not over-feeding and you can get rid of the salt in the soil by leeching it; watering the soil so that the water runs out the bottom and wash away the salt. Be sure not to leave the plant in a water tray, or it will suck the salt back up.
Sunburn occurs when a shade loving tree gets too much sun (or a tree that’s been in the shade for a long time gets moved into too much sun, just like humans). The tissue between the veins of the leaf turns yellow and then dries out and dies. This happens anywhere on the leaf, not just the edges, and affects any leaf exposed to the sun (not just older leaves like sodium damage).
Toxin in the soil kills the leaf down the vein and is often fatal. Leeching helps, but by the time you see it it may be too late.
Drought kills tissue and desiccates it so that it looks flat and dull. This happens when you let a tree dry past its wilting point (shame on you!).
Nutrient deficiencies come in several forms. Iron deficiency results in color loss in the foliage and always in the new growth. Leaves look washed out and white-ish. Manganese deficiency looks just like iron deficiency but it’s always in the old growth. The solution for both of these is to spray the tree with iron or manganese topical sprays. Zinc deficiency is often called ‘little leaf’ and results usually from too much organic material in the soil and results in small, shriveled looking leaves. You can spray the tree with a zinc topical spray to solve this.
Then there are the ‘infectious’ problems caused by organisms of some sort.
There are many funguses that affect our trees. Leaf spotting is a result of too much water on the leaves (they must stay wet for hours) and results in leaf drop. The solution here is to water your trees in the morning so that they can dry out throughout the day. As with most fungal problems (esp. those that create leaf drop), cleaning up dead leaves from around your trees will help tremendously to control the spread of the fungus.
Peach leaf curl affects plumbs, apricots, and peaches. Leaves curl up and appear to be covered in boils. When they turn white the spores are active and you are too late! Pluck the leaves before the spores are active (every single leaf!). It may take two seasons to rid yourself of the blight (but if your neighbor has peaches or apricots you?re out of luck).
Rust is a fungus that produces its spores on the underside of the leaf. These burst through the skin of the leaf on the underside and makes the leaf look rusty. Rust on Monterey pines creates galls. Rust is fond of Beech and Birch trees. Rust is dealt with by removing affected leaves and applying fungicide, again, good air circulation will help trees avoid infection.
Powdery mildews are host specific and do not need moisture and typically prefers warm days and cool nights. On oak it appears as a brownish ‘felt’ on the under side of the leaf and attacks new summer growth. One method of control is to avoid pruning in summer (which creates new growth when the days are warm). Topical fungicide can control this fungus.
Sooty mold looks like what it sounds like: black mold that rubs off as if it were soot. This is caused by insects (like aphids) that leave sugary excretions which then grow mold. Often these insects are herded by ants. The mold does not affect the plant, though it does block out sunlight. It’s more of an indicator that the tree has insect problems and should be treated with insecticide.
Fire blight appears mostly on fruit trees as a red, scorched area that appears burned and is transmitted through flowers usually by bees. Control this by removing any infected area of the tree entirely, cutting far back into non-affected areas. Dispose of the material immediately.
Camelia flower blight starts in the middle of the flower and moves outwards. This only affects opened flowers. The flower drops and deposits spores for next year’s growth. You can solve this by putting 4″ of mulch at the base of the tree and cutting off the infected flowers. In azaleas this appears as wet soggy looking flowers.
Finally, there is water mold that moves up the tree from the roots. This is caused by placing large rocks in the bottom of pots resulting in poor drainage. When we put large material at the bottom of the pot we might think we’re improving drainage, but that’s not the case. In order for water to drain through the holes in the pot, the soil adjacent to the holes must become fully saturated. When we cover the holes with large material, all of the soil surrounding the larger material must become fully saturated, which results in pockets of water that can grow fungus. Don’t put large material in the bottom of your pot, and always make sure you water your trees until the water runs through the bottom of the pot.
There are many resources online for more details on all these and other ailments. One quick search on Google for ‘bonsai fungus’ brought me to this page that lists everything here and gives greater detail on care and prevention.
There are more detailed pages on specific problems. When you encounter problems with your trees, before you react and give it more water or put it in the sun, go online and find out what’s causing the problem. More water or more sun might be the thing that puts your ailing tree over the edge.