Apricot and Almond Brown Rot

E. E. Wilson.

Probably nowhere else in the world are stone fruits grown in such variety and number as in the three States that border the Pacific Ocean. There are grown the edible varieties of peaches, apricots, the three types of cherries, plums, prunes, and almonds. There, too, are produced the numerous other species of stone fruits that are utilized as rootstocks for the edible sorts. The orchards in the region are composed not of a few trees of miscellaneous kinds grown in the back yards of scattered farm homes but many trees of the same kind in contiguous blocks extending over hundreds of acres. One entire locality may be given over to cherries, peaches, or prunes; another may be planted only to apricots and peaches; and another to almonds and peaches.

The foundation stocks of all these stone fruits were introduced into the the region. Some came from the Orient and others from Europe; some were introduced first into eastern United States and later brought west; others came directly from their foreign home.

With the establishment of the fruit industry in the Pacific coast region came disease problems: The frequent shipments of nursery stock and other propagative material provided ample opportunity for introduction of diseases. Once introduced, the diseases found large numbers of hosts to affect.

Typical of the maladies occurring on stone-fruit trees in this region are the two brown rot diseases. One of them, which I shall call apricot brown rot for convenience, was introduced from Europe. The other, which I shall call peach brown rot, apparently was native to the eastern part of the United States and developed there on the wild species of stone fruits. Whether it occurred on the Pacific coast before the fruit industry was established or was introduced later is uncertain.

The two diseases, formerly thought to be different manifestations of one malady, are produced by two closely related fungi. The one causing apricot brown rot is Monolinia (Sclerotinia) laxa. The cause of peach brown rot is Monolinia (Sclerotinia) fructicola.

The latter fungus and the disease it causes are discussed by John C. Dunegan in another article in this volume. Consequently I shall confine my discussion largely to the apricot form.

This fungus, which is common in England and on the European Continent, was found in Oregon in 19 15 and subsequently in Washington, British Columbia, and California. It is more common in California than the peach fungus. Outside of the Pacific coast region, however, it is known to occur only in a few localities in Wisconsin and Michigan.

If one reads the early accounts of brown rot in California, one is led to the conclusion that the apricot fungus occurred there long before the peach fungus. Most of those accounts deal with brown rot in the apricot and mention certain characteristics, which we recognize as those of the apricot fungus. Both here and abroad, for example, the apricot fungus is noted for its preference for the apricot. The two German investigators who described and named it mentioned this feature. Another marked characteristic is its Propensity for blighting blossoms and twigs. In California at least, the amount of fruit rotting it produces is secondary in importance to the amount of blossoms and twigs destroyed. The Peach fungus, on the other hand, seldom occurs on apricot and is distinctively a fruit-rotting organism. Its activity in blossoms and twigs is much less pronounced than that of the apricot fungus.

IN CERTAIN RESPECTS the two fungi closely resemble each other. In the form and structure of their parasitic stage on the hosts, they are indistinguishable. Taken from the host and grown on certain artificial culture media, however, they exhibit certain differences. Nevertheless, for a long time most investigators considered such differences merely the normal variations of a single fungus species. Some still believe that they should not be called different species but should be regarded as variant forms of the same species. Whether they be two species or two forms need not concern Its here; the important thing is that they behave differently with respect to disease production and host preferences. Consequently when one is present there arises a particular type of disease problem on a particular group of hosts. When the other is present there arises another type of disease problem on another set of hosts.

To illustrate how the introduction of the apricot fungus into California has affected the lot of the apricot grower, we might consider the situation in the Santa Clara Valley, which lies just south of San Francisco Bay. This valley is extensively planted to apricot, the only other stone fruit in comparable amount being the prune. The peach fungus is occasionally found there on the prunes and peaches. The apricot fungus, on the other hand, is widespread among apricot orchards and, if not controlled, causes serious damage. Consequently, each year one or more applications of a fungicide are required to hold it in check. Were it not for this fungus, therefore, the need for fungicides in the apricot orchards of this valley would be comparatively slight, since coryneum blight, the disease on which great amounts of fungicides are expended in other parts of the State, is not prevalent there.

HAVING COMPARED the two brown rot fungi, I shall now discuss in more detail the activity of Monolinia laxa, the apricot species. First, however, a word should be said about the other stone-fruit hosts of this fungus. The almond has already been mentioned as one, the others are the sour cherry, the plum, and the prune. Next, a word about susceptibility among the different varieties of these stone fruits.

Well over three-fourths of the apricots grown in California are of the varieties Blenheim and Royal, both of which are highly susceptible to blossom infection by the apricot fungus. The next most important variety, Tilton, is moderately susceptible but nevertheless is abundantly infected at times.

Investigations at the California Agricultural Experiment Station revealed some interesting effects of parentage on the susceptibility of apricot hybrids. Regardless of the susceptibility of the other member of the cross, the resistant St. Ambroise, Moorpark, and Tilton varieties apparently transmit to their progeny a large degree of resistance.

The principal almond varieties in California are Drake, IXL, Ne Plus Ultra, Nonpareil, Peerless, and Texas. Of these, the Drake is by far the most susceptible to brown rot blossom blighting. The Ne Plus Ultra and IXL are moderately susceptible. The other three varieties are highly resistant. A relatively new variety, the Jordanolo, which resulted from crossing Nonpareil with Harriett, is quite susceptible. Apparently, therefore, this almond hybrid, unlike the apricot hybrids I mentioned, did not inherit resistance from its resistant parent, the Nonpareil. Instead it inherited the susceptible qualities of the Harriett.

Among the 10 most important shipping plums, the Santa Rosa and Wick-son are the most susceptible to brown rot blossom blight. Only occasionally are plum fruits affected. Of the prune varieties, the Burton is very susceptible to blossom infection; the French (Agen) is moderately susceptible; and Imperial, Robe de Sergeant, and Sugar are resistant. In the past, most of the brown rot of prune fruit in California has been caused by Monolinia fructicola Aside from the fact that the Early Richmond and Montmorency varieties of sour cherry are known to be affected, there is little information on this host.

LET US NOW follow the life cycle of the apricot fungus through one season. In spring the fungus enters the blossoms when they emerge from between the scales of the winter buds. It grows rapidly through the blossoms and down into the supporting twigs. Soon after the blossoms wither, the fungus produces small ash-gray masses of conidia on them. Usually the conidia perform no further function, but occasionally they may cause infection of some of the fruit as it ripens. Mr. Dunegan described how the peach fungus produces spores of a second type in the infected fruit that falls to the ground. The apricot fungus has been known to produce this stage in Europe but has not been found to do so in America. Therefore the rotted apricot fruit that falls to the ground is not a source of infection the following spring. Fruits that hang in the tree, on the other hand, harbor, the fungus over the winter and in spring produce masses of conidia. Because of the sporadic occurrence of fruit rot, such fruit are relatively unimportant in California as a source of infection.

The most important sources of infection in California are the blighted blossoms and twigs, in which structures the fungus mycelium remains alive but quiescent during the hot, dry summers. After the winter rains begin, it starts a slow development. At numerous places beneath the corky outer bark the mycelium produces small compact masses by growth and segmentation. These small knots of fungus tissue, the sporodochia, or conidium-bearing structures, gradually enlarge and push up through the overlying bark to the surface. Even before the structures emerge the conidia are being produced on them. The first sporodochia appear on the twigs in early spring and continue to appear for6 weeks or more, developing in greatest numbers just before the blossoming period. Thus even before the buds open in the spring, many strategically located sources are ready to supply conidia for blossom infection.

TEMPERATURE AND MOISTURE affect both the earliness and the amount of sporodochia) development. In seasons when rains begin early in the fall and thereafter occur at frequent intervals, sporodochia may appear first in late December and continue to appear for 6 or 8 weeks, so by the time the trees blossom a very large number are present. In seasons when winter rainfall is deficient, however, they may not begin to: appear until late January and then in small numbers. Low temperature, particularly during late winter, likewise delays their appearance and reduces their numbers.

The amount of conidia that results from the infection of one blossom is extraordinarily great, because the blossoms and the blighted twig as well bear the sporodochia. Not uncommonly 30 or more of those structures develop on a single twig. Since each sporodochium produces hundreds of conidia, many thousands of the spores arise from a single infection. Multiplying this by the hundreds of infections in a tree,. one has some conception of the prodigious numbers available for blossom infection.

Conidia are washed from the sporodochia and to the flowers beneath by rain. They are also liberated from the sporodochia by air currents; being very small, they are readily carried long distances by the wind. As the air carries them through the tops of the -'trees, they are deposited on twigs, branches, and blossoms. Under favorable conditions they germinate and Produce a slender infection strand, which penetrates into the blossom tissue, thereby completing the life cycle of the fungus.