Have you — or, more precisely, one of your newly planted fruit trees or roses — ever been a victim of replant disease?

Now that spring is upon us in earnest, planting decisions are being made.  If you recently had a rose bush or fruit tree die, think twice about planting another rose bush or the same fruit tree in the vacant spot.

Replant disease is an incompletely understood phenomenon that attempts to explain the death of newly planted roses bushes and fruit trees.  If new plants were of the same species as the originals that grew there, then replant disease may explain their demise.

Planting fruit trees is similar to planting vegetables to the extent that crop rotation is recommended.  The problem has to do with a build up of disease agents, pathogenic soil fungi in particular, that can linger in the soil, in the case of trees,  for up to fifteen years.  The disease agents — fungi, bacteria, and viruses, as well as microscopic, worm-like organisms called nematodes — usually live in association with a particular fruit tree species so that you can solve the problem by planting a different one.

Where you had a lemon tree that died, for example, plant an orange instead.  Most orange trees are grafted onto semi-dwarf rootstocks so you do not have to worry about your new tree taking over the garden.  Just make sure that the rootstock used for your orange is not the same as that used for your lemon.

With deciduous fruit trees, a bit more caution is in order.  There are two broad categories of deciduous fruit: pomes and stones.  Pomes are those fruits that have a soft core, namely apples, pears, and quinces, while stones (or stone fruit) have a pit in the center and include peach, nectarine, plum, apricot, and cherry.  An interesting aside is that all species of pome and stone fruit trees are members of the rose family (Rosaceae), a kinship readily revealed in the similar appearance of their five-petaled flowers.

The best horticultural practice is to replace a pome with a stone and vice versa.  In other words, where you had an apple tree, plant a plum tree or, where you had a peach tree, plant an apple tree.  (It is extremely difficult, if not impossible, to grow pears, even low winter chill Asian pears, as well as quinces, in Los Angeles.)

I should mention that it does not always happen that a new fruit tree will die if planted in a spot where the same species previously grew.  It is more a precaution than a necessity to rotate your fruit trees, especially if the tree decline was gradual, a result of old age, and there were no signs of disease.  Apple tress, however, are different.  When a commercial apple orchard is replaced, the customary practice is to fumigate the entire orchard.  Otherwise, newly planted apple trees invariably succumb to replant disease.  So if your apple tree recently died, you should definitely be thinking about a citrus or a stone fruit tree as a replacement.

Alan Sarkisian, who gardens in Torrance, reported on a serendipitous horticultural sequence of events in a recent email.  In the same area of his backyard, Sarkisian had an ailing nectarine tree that was exuding sap and, not far from it, an apricot tree that “never produced more than three apricots in a good year.”  Sarkisian received a grape vine cutting from his uncle that he thought would grow well in full sun because he had seen those multitudinous “rows of grapevines in central California that receive full sun.”  A reasonable proposition to be sure, but you just never know when it comes to plants.  The vine did not do well in full sun so, on a whim, Sarkisian placed it between his declining nectarine tree and nearly fruitless apricot tree.

Voila!   As the trees bloomed and leafed out in the spring, the grapevine, although shrouded in shade, began to prosper.  Still in its container, Sarkisian planted it right there.  “Over the past few years,” Sarkisian continued, “the grapevine engulfed the trees.  Although I have trimmed it back at regular intervals, it has produced many bunches of grapes.”  Sarkisian ended his report with a question, “Should I keep the trees as trellises for the vine or plant new trees?”  After looking at photos of his robust clusters of grapes, all I could responsibly respond was “if it ain’t broke,

don’t fix it.”

Sarkisian brought up the matter of sap oozing from his nectarine tree, commenting that “it looks like something similar to termites is boring into the tree.”  Most often, oozing sap — gummosis is the generic term for this phenomenon — in peach and nectarine trees is attributable to a fungus or to a moth.  Where a fungus is involved, the condition is known as Cytospora canker and it is recognized as amber sap exudating from what appears to be a wound of some sort.  In fact, Cytospora canker is typically the result of physical damage to the tree, often through pruning, that creates a point of entry for the fungus.  You can remove such cankers with a sharp knife in a rather straightforward, common sense procedure.  Seach youtube.com for “Cytospora canker treatment” to see a brief video presentation on the subject.  As long as you cut away all the damaged bark, leaving a healthy margin of tissue around the wound, you should be fine.  Allow the wound to heal on its own without any additional treatment.

The moth that attacks stone fruit trees does so in its larval stage.  The larva or caterpillar is called a borer since it simply bores its way into the trunk. When a stone fruit tree is attacked by a borer, it is often at the base of the tree.  Here, the amber exudate is mixed with frass, another word for larval excrement, which has the appearance of finely milled wood.  Borers are a more serious problem than cankers and, absent chemical treatment, will quickly impact the overall health and productivity of the tree.

Termites may also munch on trees, usually at the base of their trunk, and you may see oozing sap in such cases.  The only way you know for sure that termites are involved is if you see the actually termites at work.  Keep in mind that termites are more interested in dead than living wood and, in the context of sap oozing from fruit trees, termites are much less frequently encountered than the fungal cankers and larval borers mentioned above.

Tips of the Week:  The Ojai Raptor Center advises postponement of tree trimming until fall since “nests are now active and many trees hold the invisible nests of (trunk) cavity dwelling birds.”  Of the thousand wildlife critters that the Center cared for last year, more than half of them were baby birds, the vast majority of which were eventually released.  You can learn what to do if you find a baby bird at ojairaptorcenter.org.  Thanks to Jerry Murphey for sending this notification.

Speaking of raptors, Alan Pollack,  a naturalist and gardener in Woodland Hills, reminded me that installation of a barn owl nesting box is a sensible solution for dealing with rodent problems of all kinds, from gophers feasting on roots to rats poaching on oranges.