a > inl i} * | | " r wr a Wf cA | —4 () LOO MO O1 y Rb 8. 6. 6.6:5-410:0. 6) AST CeCe SC CC CCC SS EEE SS ES ‘& ey $ ae io Oke Thesis For M.S. Annle Tres Anthracnose By A.B. Cerdley. bw fotae i A hi mn APPLE TREE ANTHRACNOSE ov by. A. B. Cordley. Some Observations on | ™~ Apple Tree Anthracnose. For several years past the apple orchards of the Pacific Northwest, including western Oregon, Washington and British Columbia, have suffered seri- ously from the attacks of a fungous disease which has been known locally as "canker," "dead spot,” or "black spot." Although of considerable economic importance, , the disease seems to have been entirely overlooked by mycologists and nothing, of importance, cOncern- ing its nature has been recorded. When it was an- nounced by Mr. Paddock, of the New York Experimaat Station at Geneva, that Sphaeropsis malorum, the cause of the well known "black rot" of the apple am quince, is also the cause of a bark disease of appb trees, it was hoped that his discoveries would ex- Plain the cause of the similar western disease, but only a cursory examination was needed to show that this is not the case. Recently I have had, with Mr. Paddock, the privilege of comparing the two 94929 diseases with the result that we were both con- vinced that they are entirely distinct. In deciding to ignore the term canker--the most commonly used of the local names which have been applied to the disease, and in proposing for it the name of Apple Tree Anthracnose, I hope to avoid confusion in the designation of the disease. in the future. The term, canker, is most commonly used in Furopean works on plant diseases to desig- nate injuries to the bark caused by the various species of Nectria; and in the eastern United States it has been applied by Mr. Paddock to a somewhat Similar disease of apple bark caused by Sphaeropsis malorum Peck. The term Anthracnose, while it has perhaps no definite botanical significance, seems appropriate from the fact th:it the fungus which causes it and for which we here propose the name Gloesporium malicorticis is closely related to numerous other fungi of economic importance which have quite generally been designated as Anthracnoses ' Apple Tree Anthracnose attacks prinecipzlly the smaller branches--those under two or three inches in diameter--although it also occurs upon the larg- Gr ones and upon the trunks of young trees. It. ap- -i- pears first in fall, soon after the fall rains be- gin, as small, irregular, brown, sometimes slightly Gepressed areas of the bark. During the fall and winter months, it spreads but slowly, but with the advent of warmer weather in spring, growth takes place rapidly until under favorable conditions the disease may invads an area several inches in diamet- er. Such areas under observation et Corvallis, Or, the past season ceased to enlarge late in May, and early in June the first evidence of spore formation was seen. At that time the diseased areas were dark brown in color, markedly depressed, and in most instances limited by ragged, irregular fissure which separated the dead from the surrounding liv- ing tissues.(see fig. 1.) Ths@e dead spots vary in size from those not more than one-half inch in diameter to extensive areas two or three inches wide by six or eight inches long. Occasionally e Single area completely girdles a branch thus kill- ing at once its distal portion; but more commonly only a dead spot occurs, from which in tne eourse of a few months the bark Sloughs off leaving an ugly wound which requires several years to heal. When these wounfis are at all numerous the branches are exceedingly rough and disfigured and are more- over greatly weakened. Early in June the first acervuli were observ- ed. They appeared as small conical elevations of the epiderm..is .nc were scattered irregularly over the diseased area. By the end of June these eleve tions had increased considerably in size and in a few instances tne overlying epidermis nad been rup- tured so as to expose tre cream colored conidial mass. Material collected at that time and taken by me to Cornell University, where it was examined about the middle of July, revealed the presence of @ few conidia none of which, however, could be in- duced to verminate. In material whim was collect- ed in July but which was not examined until early in October the conidia were more abundant but in dilution cultures in potato ager only two spores were observed to germinate. However, material which was collected at Corvalles, October 4, and which reached me a week later, had developed numer- ous conidia which germinated readily both in water and in nutrient agar cultures. It would appear, therefore, that although evidences of the formation of acervuli may be noted early in June, mature conidie are not present in quantity before August or September. Sections through a mature acervulus (see fig. 2.) show a subepidermal stroma from which arise comparatively long, closely compacted basidia on which the elliptical curved conidia are borne. As growth proceeds the overlying epidermis is ruptured and the mature conidia are set free. A true pyc- nidiam is not developed. Yhen first exposed the conidial mass is a delicate creamy tint but with age the outer surface becomes dark colored or even black. The conidia (see fig. &.) are continuous, hyaline or with a greenish tinge, elliptical, eurv- ed, coarsely granular and measure 5-7 x 16-28. Average about 6-x 2%. Late in July dilution cultures were made in neutral and in acid potato agar from material that had been collected the last of June. In these cultures not a Single colony developed. October ¢ Sithilar cultures were mace from material which was .- collected about the middle of July. In this but two conidia could be found that had germinated. So few conidia had developed that it was difficult to obtain enough for satisfactory cultures without obtaining an extensive variety of contaminating growths. To obviate this difficulty, October 6 spores from a single acervulus were carefully re- moved with a flamed scalpel, teased out in a drop of sterilized water and transferred with a steril- ized brush to marked places on plates of acid potato agar. These plates were examined Gaily anda although numerous spores were seen, none were ob- served to germinate until October 10 when two, which had made a feeble growth, were transferred to tubes of sterilized bean stems. At the time the general failure of the spores to germinate was thought to be because they had lost their vitality through hav- ing been kept too long in the laboratory; but it now appears to have been because the conidia used were not mature, since Spores from material collect- ed October 4, have continued to eermanate readily up to the middle of November. - In cell and in petrie dish cultures in potato agar, the conidia germinate readily in about twelve hours at a temperature of 22°c. At 29°, the ger- mination is retarded indefinitely, although spores in cell cultures which had been kept at this temper- ature in the thermostat for 24 and 48 hours, ger- minated as usual when removed to the lower tempera- ture. A geren tube is developed at one end of the conidium (see fig. 4) and the protoplasm begins to flow into it. Soon another tube pushes out, usually from the opposite end of the spore, and this is followed by others until from two to five tubes nave been produced. In nearly all instances there is a slignt bulbous enlargement at the origin of each tube much as is Gloesporium nervisequum #5 but > A a i - ee less marked. Growth is comparatively slow, and at 24 hours from the time of sowing, the germ tubes are rarely more than twice the lfngth of the spore. Even at this early stage, however, the production of seconad- ow ay = = Te ee me we ee : —“— o_o; «3 _— = 2 <=