APRICOT and ALMOND BROWN ROT 887 Two Brown Rot Diseases

886 YEARBOOK OF AGRICULTURE 1953 and with a nearly airtight pack or container. For prepackaged flowers or for flowers stored in water in a moderately tight cold room, the potential damage that can be done if ethylene- Apricot and producing diseased material is in- cluded is serious. Observations have indicated that once a blossom is in- Almond jured by ethylene it immediately becomes more liable to attack by Botrytis. Thus a chain-reaction type Brown Rot of response is initiated that will lead to more ethylene production and thus to more injury. E. E. Wilson The observed effects of ethylene produced by diseased plant tissues Probably nowhere else in the world emphasize the desirability of either are stone fruits grown in such variety near-perfect control or complete elim- and number as in the three States that ination of plant disease. If adequate border the Pacific Ocean. There are disease control is maintained in field grown the edible varieties of peaches, or greenhouse and in the storage nectarines, apricots, the three types of room, one of the important factors in cherries, plums, prunes, and almonds. successful long-term storage of cut There, too, are produced the numerous flowers is reduced to negligible pro- other species of stone fruits that are uti- portions. lized as rootstocks for the edible sorts. The orchards in the region are com- G. E. WILLIAMSON, assistant professor posed not of a few trees of miscellaneous of plant pathology in Cornell University^ is kinds grown in the back yards of scat- a native of Indiana. His work with dis- tered farm homes but many trees of the eases of ornamental plants began at Cornell same kind in contiguous blocks ex- in igsy. His studies on ethylene effects tending over hundreds of acres. One began with the demonstration of stimulated entire locality may be given over to ethylene production by rose leaves affected cherries, peaches, or prunes; another with black spot. He is a graduate of Wa- may be planted only to apricots and bash College and Cornell University. peaches; and another to almonds and A. W. fyiMOCK, prof essor of plant path- peaches. ology in Cornell University, has specialized The foundation stocks of all these in diseases affecting ornamental crops for stone fruits were introduced into the many years. His interest in ethylene the region. Some came from the Orient effects was a natural consequence of his and others from Europe; some were in- close association with Dr. Williamson troduced first into eastern United during his studies on ethylene production by States and later brought west; others diseased plant tissues and with Dr, C. W. came directly from their foreign home. Fischer, Jr., during his, studies on effects of With the establishment of the fruit ethylene on cut flowers in storage. industry in the Pacific coast region came disease problems: The frequent For further reference: shipments of nursery stock and other A. W. Dimock and Kenneth F, Baker: Ethylene propagative material provided ample Produced by Diseased Tissues Injures Cut opportunity for introduction of dis- Flowers, Florists Review, volume io6. No. 2'/^4, eases. Once introduced, the diseases pages 2y-2g, 1950. C. E. Williamson: Ethylene, a Metabolic found large numbers of hosts to afi^ect. Product of Diseased or Injured Plants, Phyto- Typical,of the maladies occurring on pathology, volume 40, pages 205-208, 1950. stone-fruit trees in this region are the APRICOT AND ALMOND BROWN ROT 887 two brown rot diseases. One of them, tively a fruit-rotting organism. Its ac- which I shall call apricot brown rot tivity in blossoms and twigs is much less for convenience, was introduced from pronounced than that of the apricot Europe. The other, which I shall call fungus. peach brown rot, apparently was native to the eastern part of the United States IN CERTAIN RESPECTS the two fungi and developed there on the wild spe- closely resemble each other. In the cies of stone fruits. Whether it occurred form and structure of their parasitic on the Pacific coast before the fruit stage on the hosts, they are indistin- industry was established or was intro- guishable. Taken from the host and duced later is uncertain. grown on certain artificial culture The two diseases, formerly thought media, however, they exhibit certain to be different manifestations of one differences. Nevertheless, for a long malady, are produced by two closely time most investigators considered related fungi. The one causing apricot such differences merely the normal brown rot is Monolinia {Sclerotinid) laxa. variations of a single fungus species. The cause of peach brown rot is Mono- Some still believe that they should not lima {Scier otinia) Jructicola. be called different species but should The latter fungus and the disease it be regarded as variant forms of the causes are discussed by John C. Dun- same species. Whether they be two egan in another article in this volume. species or two forms need not concern Consequently I shall confine my dis- us here; the important thing is that cussion largely to the apricot form. they behave differently with respect to This fungus, which is common in disease production and host prefer- England and on the European Conti- ences. Consequently when one is pres- nent, was found in Oregon in 1915 and ent there arises a particular type of dis- subsequently in Washington, British ease problem on a particular group of Columbia, and California. It is more hosts. When the other is present there common in California than the peach arises another type of disease problem fungus. Outside of the Pacific coast re- on another set of hosts. gion, however, it is known to occur To illustrate how the introduction of only in a few localities in Wisconsin the apricot fungus into California has and Michigan. affected the lot of the apricot grower, If one reads the early accounts of we might consider the situation in the brown rot in California, one is led to Santa Clara Valley, which lies just the conclusion that the apricot fun- south of San Francisco Bay. This valley gus occurred there long before the is extensively planted to apricot, the peach fungus. Most of those accounts only other stone fruit in comparable deal with brown rot in the apricot and amount being the prune. The peach mention certain characteristics, which fungus is occasionally found there on we recognize as those of the apricot the prunes and peaches. The apricot fungus. Both here and abroad, for ex- fungus, on the other hand, is wide- ample, the apricot fungus is noted for spread among apricot orchards and, if its preference for the apricot. The two not controlled, causes serious damage. German investigators who described Consequently, each year one or more and named it mentioned this feature. applications of a fungicide are required Another marked characteristic is its to hold it in check. Were it not for this propensity for blighting blossoms and fungus, therefore, the need for fungi- twigs. In California at least, the cides in the apricot orchards of this val- amount of fruit rotting it produces is ley would be comparatively slight, secondary in importance to the amount since coryneum blight, the disease on of blossoms and twigs destroyed. The which great amounts of fungicides are peach fungus, on the other hand, sel- expended in other parts of the State, is dom occurs on apricot and is dis tine- not prevalent there. 888 YEARBOOK OF AGRICULTURE 1953 HAVING COMPARED the two brown are resistant. In the past, most of the rot fungi, I shall now discuss in more brown rot of prune fruit in California detail the activity oí Monolinia laxa, tíie has been caused by Monolinia Jructicola, apricot species. First, however, a word Aside from the fact that the Early should be said about the other stone- Richmond and Montmorency varieties fruit hosts of this fungus. The almond of sour cherry are known to be affected, has already been mentioned as one, the there is little information on this host. others are the sour cherry, the plum, and the prune. Next, a word about sus- LET us NOW follow the life cycle of ceptibility among the different varie- the apricot fungus through one season. ties of these stone fruits. In spring the fungus enters the blos- Well over three-fourths of the apri- soms when they emerge from between cots grown in California are of the va- the scales of the winter buds. It grows rieties Blenheim and Royal, both of rapidly through the blossoms and down which are highly susceptible to blossom into the supporting twigs. Soon after infection by the apricot fungus. The the blossoms wither, the fungus pro- next most important variety, Tilton, is duces small ash-gray masses of conidia moderately susceptible but neverthe- on them. Usually the conidia perform less is abundantly infected at times. no further function, but occasionally Investigations at the California Agri- they may cause infection of some of the cultural Experiment Station revealed fruit as it ripens. Mr. Dunegan described some interesting effects of parentage on how the peach fungus produces spores the susceptibility of apricot hybrids. of a second type in the infected fruit Regardless of the susceptibility of the that falls to the ground. The apricot other member of the cross, the resistant fungus has been known to produce this St. Ambroise, Moorpark, and Tilton stage in Europe but has not been found varieties apparently transmit to their to do so in America. Therefore the rot- progeny a large degree of resistance. ted apricot fruit that falls to the ground The principal almond varieties in is not a source of infection the following California are Drake, IXL, Ne Plus spring.

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