There are several potential causes of leaf tip yellowing/dying in houseplants like Dieffenbachia (the plant pictured in the foreground). Low ambient humidity, water high in minerals (or soil with lots of mineral residue from tap water or fertilizer), over-fertilization, under- and over-watering, and pest damage (spider mites, often) are all possible factors.

You can inspect the leaf undersides for indications of mites by looking for fine stippling (pale flecks on the green areas of the leaf), especially around main leaf veins or the leaf edges. Silken webbing that gives spider mites their namesake won't always be evident unless populations are very high. Management options for spider mites are included in the page linked above. We don't recommend using a pesticide if mites are not detected.

If the plant is in a pot that doesn't drain, move it into one that has at least one drain hole. If it is in a drained pot, make sure the saucer it sits on is emptied promptly after watering, so the pot doesn't sit in pooled water and reabsorb too much, which may kill roots. Similarly, when watering, let the potting mix dry out a bit between watering. It should feel somewhat dry to the touch about an inch or two below the surface before the plant should need to be watered. Leaf tips and edges can dry out if the roots get too dry for too long (not all plants wilt readily as they start to dry out), but they can also do so when too wet. This is because roots that are too wet can't get the oxygen they need, and they die back; since there are fewer healthy, functioning roots at that point, the foliage suffers from drought because they can't access enough water, which is also why over-watered plants can wilt. (Your plant isn't wilting, but wilting is a more advanced stage of drought or over-watering stress.)

Has the potting mix been replaced in the past year or two? If not, salt residue can damage roots, which in turn causes leaf symptoms. Salts in this context are not necessarily sodium (but could be, if you happen to use water from a source using water softener). Instead, they are any mineral residue that builds up over time from regular tap water (or bottled water), plus any fertilizer the plant was given that it didn't use. By putting plants into fresh potting mix every year or two, that resets the salt levels where roots won't be stressed. It can also help improve the drainage in old potting mix, which tends to hold too much water and not enough air for root health.

If low humidity is the cause (generally more of an issue in winter, though air conditioning does dry out the air too), putting the plant outside for the summer might help. If you have the space to do so, put it in shade, since the foliage will not be adjusted to receiving any direct sunlight (which is stronger outside than what comes through a window). The photos suggest the plants are already outside, but in case they're in a sunroom or a location that's not receiving ambient outdoor (and humid) air, we mentioned it just in case.

If the plants are outside but were only recently put there from a location inside (or from a store), they are still acclimating to new environmental conditions, and subsequent new growth should look better. Sunscald and other minor damage symptoms can occur when leaves have to adjust to brighter light or warmer temperatures, for example.

Whichever issue may have caused these symptoms and then successfully resolved, the damaged leaves cannot heal (so they won't look much better), but an indication of improvement would be new leaf growth that does not keep turning yellow or brown on the tips/edges.

If none of the above situations seems to apply to these plants, we can try another diagnosis, but will need more close-up photos of the symptoms (top and bottom leaf surfaces) to help narrow-down what might be going on. More information about where the plants are being grown, when the symptoms appeared, and if any plants other than Dieffenbachia are being affected will be also helpful.

Miri