From the Director’s Desk: Q&A, Part III

In this last Q&A ses­sion, I am focus­ing on leaf col­or change dur­ing the spring and sum­mer grow­ing sea­son, not autumn. When you see leaves chang­ing col­or out of sea­son, it is time to do some detec­tive work. Among the rea­sons why plant leaves turn col­or are envi­ron­men­tal con­di­tions, pests, and dis­eases. This change in col­oration may also be a nor­mal process, rather than a problem.

Q: ​“Why are my plants leaves turn­ing brown?”

A: There are many fac­tors that can cause leaves to turn brown. One rea­son is water stress indi­cat­ing the plant is not receiv­ing enough water, espe­cial­ly dur­ing the sum­mer heat. Typ­i­cal­ly, these leaves show the brown­ing at the tips and edges first; if the drought con­tin­ues, the leaves begin to dry and die. This is some­times referred to as envi­ron­men­tal leaf scorch. If diag­nosed ear­ly, drought stress can be reme­died by prop­er water­ing. It should be not­ed that less fre­quent deep water­ings are pre­ferred to more fre­quent light water­ing, which caus­es the plant to devel­op shal­low roots. Alter­na­tive­ly, too much water­ing or poor­ly drained soils can cause a root rot and this will result in the entire plant turn­ing brown, los­ing leaves, and even­tu­al­ly dying. 

Brown blotch­es, spots, or streaks on the leaves may be a sign that the plant has a fun­gal or viral prob­lem. A few com­mon exam­ples are ​‘black tar spot’ found on some maple trees and the brown spots on apple tree leaves. Both are caused by fun­gal pathogens that cause unsight­ly aes­thet­ics and pre­ma­ture leaf drop. The good news is that in both cas­es these dis­eases do not kill healthy trees. A good con­trol is to remove dis­eased leaves as they fall and dis­pose of them off site. Some insects can also cause brown­ing of leaves, so care­ful inspec­tion to deter­mine the cause may be need­ed. Last­ly, brown leaves can be caused by fer­til­iz­er burn, which would show up first towards the leaf tip and margins.

Q: ​”Why are the leaves on my plants turn­ing yellow?”

A: Some old­er leaves nor­mal­ly turn yel­low, but yel­low leaves may also be a sign that some­thing isn’t right. Most com­mon­ly, the cul­prit is from too much water — or not enough. If plants do not receive enough water, they will drop leaves to help con­serve resources. Too much water, espe­cial­ly in poor­ly drained soil, will cre­ate stress on the plants. Plants that are moved out­doors in spring may turn yel­low due to too much light inten­si­ty. If you notice this, place them in a shady area to recov­er before expos­ing them to light again. The good news is most of these plants will recov­er, although they might expe­ri­ence a setback. 

A nitro­gen defi­cien­cy is also some­times a cause for yel­low­ing leaves dur­ing the active grow­ing season.

As the end of sum­mer nears, you may see some leaves turn­ing yel­low, which is nor­mal as the plants pre­pare for win­ter dor­man­cy. Yel­low­ing is also seen on the inte­ri­or nee­dles of conifers. This is a nat­ur­al process as new nee­dles emerge. How­ev­er, if your ever­green or conifer is los­ing the inte­ri­or nee­dles and they are not grow­ing back, caus­ing a sparse canopy, the cul­prit would most like­ly be a nee­dle cast disease.

Q: ​“Why are my plant leaves turn­ing black?”

A: If your plant leaves have a black sub­stance coat­ing them, it is most like­ly a fun­gus called sooty mold. This mold is typ­i­cal­ly found in con­junc­tion with plant pests such as aphids, which secrete a sweet sub­stance called hon­ey­dew that the sooty mold likes to grow on. The sooty mold is not tox­ic to your plants, but is unsight­ly and may get on struc­tures such as cars and hous­es. To reduce sooty mold, take steps to con­trol the aphids on your plants. Aphids are rel­a­tive­ly easy to con­trol with envi­ron­men­tal­ly friend­ly insec­ti­ci­dal soaps and oils. 

Q: ​“Why are my plant leaves turn­ing white?”

A: White leaves are most like­ly caused by the col­o­niza­tion of fun­gi, which cre­ates a fun­gal dis­ease called pow­dery mildew. High humid­i­ty pro­vides ide­al con­di­tions for pow­dery mildew. Increas­ing the sun­light or air cir­cu­la­tion can con­trol it. To increase light, thin out plants by prun­ing; ade­quate spac­ing between plants will help with air cir­cu­la­tion. Do not over­fer­til­ize. Treat infect­ed plants with an organ­ic fungi­cide con­tain­ing sul­fur. In addi­tion, many gar­den­ers use home reme­dies such as a bak­ing soda solu­tion (1 Tbl bak­ing soda and ½ tsp of castile soap (non-deter­gent) mixed into 1 gal­lon of water), which is then sprayed on the plant as a pre­ven­ta­tive and/​or treatment. 

Q: ​”Why are my plant leaves turn­ing purple?”

A: Gen­er­al­ly speak­ing, a phos­pho­rous defi­cien­cy may be the cul­prit. To inves­ti­gate, I advise check­ing the pH lev­el of the soil. Most plants pre­fer a slight­ly acidic soil rang­ing from 6.2−6.8. This is impor­tant because the nutri­ents may be in the soil, but with the wrong pH, the plant is unable to uptake and uti­lize them. If the pH seems in the accept­able range for your plants’ require­ments, then apply­ing a com­plete fer­til­iz­er that has ade­quate phos­pho­rous should solve your problem. 

As a mem­ber priv­i­lege, ques­tions can be emailed to me at fred@​landisarboretum.​org, any­time. Please include a pho­to or two. To learn more about becom­ing a mem­ber of Lan­dis Arbore­tum, please vis­it our web­site at www​.lan​dis​ar​bore​tum​.org.