DAXGEROUS INTERNATIONAL FOREST TREE DISEASES 115 



Literature : 



Davidson, R. W. Vrwula cmterium is possibly the perfect stage of 

 Strumella coryneoidea, Mycologia 42 : 735-742. 1950. 



Heald, F. D., and Studhalter, R. A. The Strumella disease of oak 

 and chestnut trees. Pa. Dept. Forestry Bui. 10, 15 pp. 1914. 



Hughes. S. J. Microfungi V. Conoplea Pers. and Exosporium Link. 

 Canad. Jour. Bot. 38 : 659-696. 1960. 



Dwarfmistletoes of Conifers 



Job Kuijt 



Department of Biology and Botany. University of British Colum- 

 bia, Vancouver, British Columbia 



Arceuthobium spp. are conifer-inhabiting flowering plants of the 

 mistletoe family (Loranthaceae). Leaves are reduced to scales, op- 

 posite, decussate, inconspicuous, same olive-green color as the stem. 

 The internodes are round or rectangular in cross section, depending on 

 species and age. 



The larger species (A. campylopodum and A. vaginatum) are com- 

 monly up to 10 cm. long, while the smaller ones (A. douglassi and A. 

 pusillum) "are usually no more than 1.5 cm. The plants are unisexual. 

 Staminate flowers are about 2— i nun. in size, 3- or 4-partite. deciduous, 

 anthers sessile with ringlike archesporium around a central columella, 

 pollen tricolpate. spinulose. Pistillate flowers are about 1 nun. in size, 

 little differentiated, with two perianth segments greatly reduced, lip- 

 like around the short and blunt stigma. Fruit at maturity is a re- 

 curved, turgid berry, ovate to elliptical in shape, with a conspicuous 

 transverse color demarcation. 



The fruit is explosive, releasing the single seed upwards or obliquely 

 away for many feet. The seed, which consists of a single cylindrical 

 and largely undifferentiated embryo surrounded by a large amount of 

 chlorophyll aceous endosperm, germinates on the surface to which it 

 has become attached. If this surface is a young branch of a suitable 

 host, the radicle penetrates into living tissue and a new individual 

 develops. Since the remainder of the seed dies or falls away, dwarf- 

 mistletoes have a completely internal stage which may last for more 

 than a year. Flowering time varies for the different species, but ex- 

 pulsion of the seed always takes place in the fall. 



The haustorial or endophytic system is a complex, ramifying system 

 divisible into a longitudinal system of strands, external to and more or 

 less parallel to the host cambium, and a radial system consisting of 

 sinkers radially oriented in phloem and xylem, such sinkers taking 

 their departure from cortical strands. Direct xylem-to-xylem connec- 

 tions are common between the largely vascular older sinkers and the 

 host tracheids. Cortical strands and perhaps all sinkers originate as 

 uniseriate filaments of cells each with a single apical cell, but divisions 

 in the more distal cells turn the strands into vasculated structures 

 much like normal roots. 



Symptoms of the disease vary not only between species of dwarf- 

 mistletoe, but also within one species when several host species are 

 involved. The simplest response of the host is a localized, somewhat 

 fusiform swelling. In this case the endophytic system is confined to 



