May 12, 1917 



HORTICULTURE 



615 



Ti-E HOLLOW STEM DISEASE OF PAEONIES 



BY WILLIAM ROLLINS, BOSTON. 



Through roots received from a 

 paeony specialist in Pennsylvania my 

 paeony garden was threatened with 

 ruin. It therefore seems desirable to 

 furnish gardeners with data that will 

 enable them to recognize this disease 

 as soon as it appears in the garden. It 

 is hoped also that the illustrations will 

 enable them to detect the fungus in the 

 roots before planting. Figure 1 shows 

 the effect of Hollow Stem on a paeony 

 root. It produced the small stems and 

 buds. This root was chosen because it 

 also showed another disease, Knob 

 Root, which caused the nodular condi- 

 tion. The latter has been a not uncom- 

 mon disease for some years, and its 

 treatment is within our present knowl- 

 edge. 



Hollow Stem is a much more serious 

 disease, and more difficult to recog- 

 nize, and Its treatment is not so well 

 known. 



When a box of paeony roots is re- 

 ceived from the dealer open it on a 

 piece of old sail cloth at a distance 

 from the garden. If any of the stalks 

 are dead and hollow at the level they 

 were cut in preparing the root for ship- 

 ment the plant probably has the Hol- 

 low Stem fungus. Look with a hand 

 magnifying glass for small dark raised 

 spots, especially inside the stems that 

 are hollow. Figure 2 shows an affect- 

 ed stem cut half through, to show the 

 inside surface. The picture is enlarged 

 about as much as the stem would be 

 with a hand magnifier, such as should 

 always be used in examining every 

 paeony root before planting. If black 

 spots are found, cut off the stem low 

 enough to be sure the stump is healthy 

 and paint the cut surface with a fungi- 

 cide. Make sections of the amputated 

 parts and examine them. The paeony 

 has many other diseases not so serious 

 as Hollow Stem, and unless the micro- 

 scope shows the Hollow Stem fungus, 

 the subsequent treatment does not 

 need to be so radical. I assume that 

 every gardener has a microscope, for 

 gardening is one long struggle with 

 minute enemies. A very inexi)ensive 

 instrument will serve. All the micro- 

 photographs used to illustrate this pa- 

 per were made with an instrument the 

 optical parts of which cost $14.50, or 

 less than the price of a single root of 

 many varieties of paeonies. If the 

 fungus is found, destroy all the roots of 

 the common varieties affected. With 

 the more valuable roots cut until sound 

 tissue is reached. Though the roots 

 are divided into pieces each will grow 

 if it has one healthy bud. Gather up 

 all affected pieces and burn them with 

 the packing case and contents. Figure 

 3 is from a mature root of the variety 

 Therese. cut to show a hollow in the 

 root caused by hollow stem fungus. 

 From an ideal root it became a ruin in 

 two years. The plant never showed 



the least sign of Knob Root, to which 

 a number of paeony growers attributed 

 its ill health. Observe the small buds 

 at the bases of the former large stems. 

 If this plant had been left in the 

 ground, this year it would have pro- 

 duced only feeble stems, which would 

 have developed buds that would not 

 have opened. Wlien in health this root 

 produced flowers twenty-two centime- 

 tres in diameter. Figure 4 is a top 

 view of the same root, with the hollow 

 stems cut at the bases to show how lit- 

 tle tissue remained to make new stems. 

 The healthy tissue is white in the pic- 

 ture. In 1914 this was a perfect speci- 

 men of a paeony root. 



Before planting the roots on which 

 the surgical operations have been per- 

 formed, immerse them in commercial 

 formalin, or in a deep red solution of 

 permanganate of potash and thorough- 

 ly brush all parts except the buds, with 

 the solution. The next spring, if any 

 of the stalks wilt, make sections and 

 hunt for the fungus. Later watch for 

 the new spores. The first sign will be 

 a minute red spot, where the spore 

 has turned the chlorophyl of the cells 

 to an autumn tint. As the diseased 

 spot enlarges, the centre becomes the 

 color of dead wood. Figure 5 shows a 

 paeony stem with the fungus in the 

 early stages. It is enlarged about as it 

 would be by a hand magnifier. Several 

 paints of attack are shown, from one 

 the size of a pin head at A to others 

 in more advanced stages at B, C, D. 

 From A a single fruiting stalk of the 

 fungus is arising. Figure 6 shows the 

 fungus in the stage where it has sent 

 up hundreds of fruiting stems from 

 the bark of a stem. The spore heads 

 are about a millimetre above the sur- 

 face of the stem. At the left many of 

 the stems have shed the spores, be- 

 come black, and under the microscope 

 look like bits of graphite. Figure 7 is 

 a microphotograph of the fungus in the 

 fruiting stage. In this and the follow- 

 ing illustrations the real size of the 

 subjects can be determined by the ac- 

 companying scale, which is a photo- 

 grapli of a Rogers micrometer, with 

 divisions of one-tenth and one one- 

 hundredth of a millimetre. 



Each of the wide spaces in figure 7 

 is one-tenth of a millimetre. In all 

 subsequent pictures each division of 

 the scale is one one-hundredth of a mil- 

 limtere. This picture is a thin bit of 

 the outer tissues of the bark of the 

 stem and the dark masses are the root 

 clumps of the fungus from which the 

 fruiting stems arise. The specks in 

 the background are the spores that 

 flew off in making the section. Wliile 

 too small to be individually seen by 

 the unaided eye they are when ripe, so 

 easily detached by a slight touch of 

 the paeony stem, that they arise as a 

 plainly visible cloud. This is the 

 reason why the disease can spread 

 over a large garden so rapidly, for 

 being carried by the wind, even in wet 

 weather they reach distant parts with 

 ease. They are as transparent as 

 glass. To study their structure they 

 must be killed instantly and stained. 

 When the wind blows a spore against 



a paeony it develops quickly. Figure 

 8 shows a spore that has penetrated 

 the tissue of a paeony stem and Is 

 sending out the first bud. Figures 9 

 and 10 further development in the cel- 

 lular tissue of the paeony stem. The 

 nearly black bunches are the enlarge- 

 ments that come in the fungus roots 

 at points where it will send up the 

 stems into the air that will make fruit- 

 ing plants. Figure 11 shows one of 

 these stalks in an early stage before it 

 begins to branch. Notice the tip where 

 the first spore is beginning. Figure 12, 

 an aerial stem that is branching and 

 developing a few spores. Figure 13, a 

 fully developed aerial stem with the 

 spores in the mature state ready for 

 the wind to send them flying over the 

 whole garden. 



Figure 14 is most interesting, as it 

 shows a single cell of the stem of a 

 ))aeony in perspective. The mottled 

 snake like thing is a fungus root. It 

 has pushed through the wall between 

 the cell and the one next and after 

 sucking out the life of the cell, destroy- 

 ing its structure, it is attacking the 

 nucleus which is the most resistant of 

 the tissues. 



Figure 15 shows a similar cell where 

 the fungus has nearly destroyed the 

 nucleus. Figure 16 shows the stage 

 where the fungus roots have destroyed 

 practically all of the tissue of the 

 paeony stem, the whole picture being 

 only a mass of fungus roots running in 

 every direction. When this stage Is 

 reached which may be in a week after 

 the spore strikes the paeony, the stem 

 wilts, and the gardener begins to won- 

 der what is the trouble with his plant. 



Treatment, 



Soak the ground with a solution of 

 permanganate of potash, and drench 

 the plants with it from time to time. 

 During the stage when the fungus is 

 fruiting, paint every suspicious spot on 

 the paeony with tar. Do not use a 

 fungicide that is not sticky for in ap- 

 plying it the jarring of the plant will 

 shake off the spores before they are 

 killed. Remove all affected parts that 

 are not necessary for the development 

 of strong buds for the next season and 

 steam them. A stock feeding outfit is 

 suitable, if it has a boiler in which 

 steam can be developed. From it run 

 a pipe to a barrel and throw the affect- 

 ed parts of the plants into the barrel 

 partly filled with water having a fungi- 

 cide in it. Once in two or three days 

 steam thoroughly and remove the 

 steamed tops and stems to the com- 

 post pile. In the autumn when the 

 tops are removed, cut them very low, 

 gather them in iron bushel baskets, 

 and burn them. After the stems are 

 cut observe the stumps left on the 

 plants and if any of them are hollow 

 they probably contain the fungus in 

 the resting stage already described. 

 Dig down and cut off all such stem un- 

 til you come to sound tissue. Paint the 

 cut surface with a fungicide and cover 

 with earth. Paeony plants should be 

 six feet apart to allow a good circula- 

 tion of air to promote dryness as the 

 fungus grows best in damp air. 



