Tricks I've Learned About Diagnosing Plant Problems:
- Just about everything can cause yellowing leaves so it is almost never a reliable diagnostic symptom beyond the plant being unhappy. Causes of yellowing include: over-watering, under-watering, too much light, too little light, too hot, too cold, nutrient deficiency, pests, and being root bound. Ditto on dropping leaves, which is really just a later stage of stress.
- If there's marks on new growth or twisted new growth alongside yellowing leaves or leaf drop you have pests... but it may take another week before the infestation starts to show... consider quarantining the plant immediately upon seeing this mixture. Early stage infestations can look a lot like under-watering or nutrient deficiencies. (Edit: treat neighboring plants to any signs of pests, as pests spread quickly... a visible infestations will rarely be remedied by quarantine alone)
- The only other cause of damage spots on new growth (as opposed to discoloration) is usually sun-burn, but new growth may eventually grow out healthy if the plant is properly hardened off to light. This is actually more a matter of leaves on the top of the plant will sustain the damage, but it also looks different from pests.
- Sun-burn generally doesn't cause holes, but leaves can split if they're growing too fast - I have seen this on my watermelon peperomia, who also shows yellowing when exposed to too much light - some Haworthias also yellow when exposed to too much light, but they will also yellow with too little light or too much water.
- Most plants will sacrifice old growth to protect new growth from either over or under watering. Sacrifice of new growth is almost always an issue of under-watering (I have only noticed this on few plants: peperomias, strawberry begonias (which will occasionally sacrifice the newest plantlets on their stolons and progress to pulling water from old growth on the mother plant), and Hoyas (which will sacrifice their tendrils before their leaves).
- Necrotic (dry dark/brown) patches with yellow rings are often caused by over-watering or splashing water onto the leaves (later of which often seen on African Violets and the likes). Sun-burn can look similar but usually lacks the bright yellow boarder.
- Fiddle Leaf Figs will drop leaves for any upset - for under-watering the leaves will begin to lose their shine before they drop. Over-watering can show up with edema (on figs this is little red splotches in the early stage) on new growth with curled leaves, progressing to lower leaf loss. Bugs can look like a mixture of these, but will show up with holes in new growth.
- Aloes, Haworthias, Gasterias, Sansevierias, and Sempervivums get dry tips when under-watered as well as thin, bendy (but not soft or mushy) leaves. Over-watering appears as yellowing and swelling progressing to mushy and rot. Aloe veras are surprisingly tolerant of over-watering and can get pretty plump and bright, lime green before succumbing to rot, which usually comes up the center of the rosette, effectively killing the plant.
- Sedums, Pachys, Graptos, and Echeverias are a pain in the butt for the same reason as Figs. Some will show wrinkling when under-watered but some will just cannibalize their lower leaves for water, resulting in yellowing, shriveled leaves that progress to dried up and dropping. Over-watering can look very similar but in early stages will show up with swelling leaves and in late stage can result in soft, mushy leaves and stems. Plump, healthy leaves will also easily pop off when touching an over-watered plant. If there's rot in the stem they can usually be cut and propped, if it's extensive any leaves that still feel healthy can usually be propped if the stem can't be saved.
- Scindapsus will curl their leaves for both under-watering and over-watering, but it's more dramatic when under-watered and over-watering will also show up with edema with yellowing/browning patches. Lower leaves will dry with under-watering.
- Philodendron micans and ferns (inc. asparagus fern, though they're not a true fern) will stop growing when over-watered and sacrifice lower leaves in progression.
- Begonia maculata will drop leaves anytime it's pissed off and at the smallest outbreak of pests (it only takes one for relatively profuse leaf drop).
- Dieffenbachia get yellow spots progressive to necrosis when over-watered. Pests will cause leaf-tip burn that includes a loss of color progressing to dry, brown tips that will creep up the leaves. I suspect that under-watering will show the same way.
- In moisture loving plants (like begonias, large philodendron varieties, Colocasias, Avocados, ect), especially those with large leaves, over-watering or compacted soil will often result in an exchange of "grow a leaf, lose a leaf" along with dark, necrotic spots of leaf edema.
- Peperomias and Calla Lilies can do the "grow a leaf, lose a leaf thing" as well but they also get soft, mushy stems rather than leaf edema.
- Most plants with soft, thin leaves will flop over suddenly and dramatically like they're about to die when they're under-watered - leaves will feel thin and lifeless - but will prop up usually within an hour of watering. Over-watering in these plants usually result in a slow, progressive droop with a plant that looks increasingly more sad - end stage usually results in yellowing and leaf drop on old growth.
- Cyclamen (which have succulent leaves and stems) will droop in both leaves and blooms when under-watered. When infested with pests this will be pared with sacrifice of new blooms rather than droop on mature blooms. No matter what you do they will go through leaf loss on old growth, I've come to the conclusion this is just how this plant does... it just does so more often than other plants in my experience.
- Hoyas and jungle Cacti get soft/bendy, thin, wrinkled leaves when under-watered.
- Seedlings usually yellow and collapse due to too much water. While they're a little more tolerant of extra moisture, extra moisture in conjunction with too little light (or just way too much water) will cause their demise.
- Sometimes the reason succulents are getting stretched out, especially when you're giving them tons of light, is because they're being fed too much. Stop fertilizing or consider cutting their soil with more sand or perlite/pumice to reduce their nutrients if they're not growing as compact as you expect them too. Over-fertilizing can also surprisingly result in a stretched out succulent with little leaves (too much P or K, as succulent fertilizers tend to be higher in these nutrients) even in the face of light levels that would readily burn other houseplants.
- You will almost never have a problem with a houseplant getting too cold unless your heat goes out or their leaves are making direct contact with a window that has ice build up on it. Heat is more likely if you're growing something that prefers cooler temps
- Cold-loving crops will flop over if not given cooler night temps DURING growth. Upon transplanting outside, old growth will not begin to stand up, but new growth will. Despite this, you can grow some cold-weather crops indoors but they will be less sweet than when grown outdoors and the plants will never look quite healthy.
- Succulents often show being totally root bound by losing their color and appearing under-watered. Other plants will generally also show signs of under-watering or suddenly become very thirsty.