igii 



BETTER FRUIT 



Page 41 



ROOT DISEASES CAUSED BY ARMILLARIA MOLLEA 



BY W. H. LAWRENCE, WESTERN WASHINGTON EXPERIMENT STATION, PUYALLUP, WASHINGTON 



DURING the past three years a 

 large number of inquiries have 

 been received concerning the 

 death of individual fruit trees among 

 m.any apparently healthy ones, also 

 groups or hills of bush and other small 

 fruits. The examination of many of 

 these dead or dying plants has revealed, 

 in many instances, the presence of dark 

 colored, cord-like bodies (rhizomorphs) 

 on the stems and roots. After noting 

 the nature of the injury caused by these 

 rhizomorphs in numerous plants of vari- 

 ous sorts, and by comparing the rhizo- 

 morphs with those of Armillaria mellea 

 collected by Piper and Fletcher in 

 Clarke County, and the rhizomorphs of 

 the same species of toadstool collected 

 by the writer in New York, the fungus 

 was concluded to be a form of Armillaria 

 mellea. At a later date a few mature 

 toadstools were collected. By compar- 

 ing these specimens with the illustra- 

 tions and descriptions of Armillaria mel- 

 lea and with Armillaria mellea bulbosa 

 by the above mentioned authors the 

 writer is of the opinion that at least four 

 forms of Armillaria mellea exist in this 

 state. Under various conditions all of 

 these forms are more or less parasitic, 

 and are the cause of numerous root 

 diseases. 



It is impracticable to separate the four 

 forms into different varieties, since the 

 botanical differences are so indistinct 

 and variable that almost any one form 

 would do equally well for a type speci- 

 men. The four forms mentioned include 

 the two discussed in the above cited bul- 

 letin. The other forms occur in the 

 Puget Sound country. One variety is 

 delicate, small, light brown, bearing 

 rather slender but very promiscuously 

 branched and somewhat flattened dirty 

 white to light brown rhizomorphs, which 

 cover the greater portion of the sub- 

 stratum. This form has been collected 

 on two occasions growing on dying 

 blackberry cane. It will not be men- 

 tioned again in this bulletin. Since the 

 two above named forms have been dis- 

 cussed at some length in a previous 

 bulletin they will not be mentioned 

 further. The more destructive form of 

 those occurring in the Puget Sound coun- 

 try and the one discussed at some length 

 in this article is intermediate in form 

 between Armillaria mellea and Armil- 

 laria mellea bulbosa. 



Armillaria mellea is one of the many 

 kinds of low form of plant life which are 

 obliged to take their sustenance from 

 living animals, or plants, or decaying 

 vegetable or animal matter. It consists 

 of two parts — the vegetative and the 

 reproductive. 



The vegetative part consists of numer- 

 ous thread-like strands called the myce- 

 lium. The mycelium is the sole cause of 

 all the injury which the toadstool causes. 

 It grows into various substances in order 

 to collect food. Many times these 

 threads penetrate the roots and stems of 

 living plant*. They may be confined to 

 one or more spots of variable extent, or 

 may infest the entire body. In many 



cases they are not visible to the naked 

 eye, but under some conditions they 

 become abundant enough to form white 

 layers. Throughout more or less exposed 

 places, and especially in the soil, the 

 threads collect in cord-like strands and 

 are enclosed by a thick black or brown- 

 ish wall. These bodies are the rhizo- 

 morphs. The mycelium and rhizomorphs 

 constitute the vegetative part. The vege- 

 tative portion has an odor similar to that 

 of the edible mushroom. 



The fruiting bodies, which are toad- 

 stools, form on the mycelium and rhizo- 

 morphs. As the vegetative portion grows 

 it collects an abundance of food. Minute 

 button-like bodies, which are the young 

 toadstools, form on it. With an abun- 



dance of food in the mycelium these 

 toadstools mature in size in a very short 

 time. A mature toadstool consists of a 

 stalk with a cap on the top. On the 

 under side of the cap numerous curtain- 

 like projections occur, which are known 

 as gills. They are arranged around the 

 stem similar to spokes in a wheel. On 

 either side of each of these curtains 

 numerous spores are produced. When 

 the spores are ripe they fall to the 

 ground. They are distributed by various 

 agencies. Being very light in weight, 

 they are undoubtedly carried long dis- 

 tances by the wind. When the spores 

 lodge in places favorable to germination 

 they produce small thread-like bodies, 

 which, if a supply of food can be 



Figure 5— SHOWING ALL PART OF ARMILLARLX MELLEA VISIBLE TO 

 THE NAKED EYE 



A, Fruiting bodies or toadstools; B, Toadstools with one form of the vegetable portion 

 (rhizomorph) attached; C, The mycelium, another form of the vegetable portion. 



