Fungous Diseases of Ontario Orchards: Pear Blight 



M. B. Waite, Pathologist in Charge, Investigation of Diseases of Fruits, U. S. Department of Agriculture 



THE well known bacterial pear blight 

 caused by a tiny bacillus is one of 

 the most prominent orchard dis- 

 eases in Ontario. It attacks particu- 

 larly the pear and the apple, but also 

 affects the quince, the Siberian crab 

 apple, the wild crab apple, the haw- 

 thorns, and practically all the fruits of 

 the pome family. The germs producing 

 the disease enter the tree in three differ- 

 ent ways: First and most commonly, 

 through the blossoms, being distributed 

 from flower to flower and tree to tree 

 very widely through bees and other 

 flower visiting insects ; second, through 

 tender tips of growing shoots, including 

 the water sprouts at the bases of the 

 trees ; and third, directly into the fleshy 

 bark. Infections of this latter type are 

 few in number but result in a very serious 

 form of the blight. The amount of da- 

 mage resulting from an infection may 

 vary enormously. Thousands of infec- 

 tions simply kill the blossom cluster or 

 a few inches of the tip of a growing 

 twig. On the other hand the blight may 

 run down on to large branches or run 

 clear to the ground, killing the whole 

 tree, or it may spread from a lateral 

 infection, doing more or less damage. 

 The spread of the disease, the number of 

 infections and the amount of extension 

 of the blight on a tree after infection are 

 dependent upon a number of different 

 conditions. The factors controlling an 

 outbreak of pear blight may be summed 

 up as follows : 



First. — The presence of the germ and 

 the amount of holdover blight available 

 for reinfection. 



Second. — The amount of bloom on the 

 trees. It is difficult for young orchards 

 to catch pear blight until they blossom. 



Third. — The number of insect visitors 

 available. This is largely, however, con- 

 stant if the next is favorable. 



Fourth. — The weather during blossom- 

 ing time. If the weather is favorable for 

 insect activity and nectar secretion, the 

 blight germs are generally carried about. 

 On the other hand rainy weather or cold 

 dry sunny weather discourages the 

 spread of blossom blight. 



Fifth. — The variety and species of the 

 tree. Each different horticultural variety 

 and each species of pomaceous fruit has 

 a different relative resistance to the 

 disease. 



Sixth. — The age of the tree. Young 

 trees are more susceptible than older 

 ones. The most susceptible age comes 

 at the time the trees are first in bearing, 

 say the first four or five years they are in 

 bearing. 



Seventh. — The vigor of growth. This 

 is influenced by fertility of the soil, soil 



moisture conditions, favorable weather, 

 artificial manuring, fertilizing and culti- 

 vation. In general, those conditions 

 most favorable to vigor of growth of 

 the tree are most favorable to the blight, 

 and conversely those influences which 

 dwarf or check the growth of the tree 

 tend to hinder the progress of the blight. 

 Most of the blight dries out in the trees 

 during the summer. On the other hand, 

 occasionally at the bases of the blighted 

 twigs or more commonly on the thick, 

 fleshy bark on the large limbs and on the 

 bodies of the trees, the blight keeps 

 slowly progressing until the close of the 

 season. The germs die out of the dead 

 bark but keep alive on this advancing 

 margin and the cool, moist weather of 

 winter, though it checks their growth, 

 tends to keep them alive until spring. 

 This type of blight we' call "holdover 

 blight" and is the source of the new 

 infections each season when the trees 

 spring into growth. 



COLLAR BLIGHT 



I wish to call particular attention to 

 the forms of pear blight known as body 

 blight and collar blight. Body blight 

 occurs abundantly on young trees, more 

 particularly on young Bartlett and other 

 pear trees, but it may even kill young 

 apple trees. Frequently the blight gets 

 into the thick fleshy bark on the body of 

 young trees either by running in through 

 a fruit spur or water sprout or by means 

 of the punctures of insects or mechani- 

 cal injuries. Possibly the germs may 

 enter directly into the fleshy bark 

 through growth cracks. Once in the 

 fleshy bark the germs rapidly spread in 

 all directions. They extend vertically 

 in the direction of the vessels and fibres 

 more readily than they do laterally. Of- 

 tentimes the infection is at the ground 

 line or at the collar of the tree. Some- 

 times even below the soil line. The 

 blight then spreads on the moist soil in 

 all directions. Below the soil it more 

 readily spreads in a lateral direction 

 around the collar and down the bark of 

 the roots. Frequently large irregular 

 areas are formed and quite often they 

 girdle the trees, absolutely killing them. 

 Sometimes a V-shaped area runs up 

 from the point of infection on to the 

 body. French stocks are often more sus- 

 ceptible to blight than the grafted top 

 even though the latter may be the sus- 

 ceptible Bartlett. 



Trees affected with body blight usually 

 take a year to die. This is unlike the 

 branches, which are, of course, killed by 

 the blight of the current season. Trees 

 often live another year, and sometimes a 

 second, even though completely girdled, 

 and show the destructive effects the sea- 



son after the blight has done its work. 

 Sometimes the blight germs have died 

 entirely out of the bark and the dead 

 bark has dried up and yet the tree dies 

 the following year from girdling. 



In case of collar blight, where it does 

 not completely girdle the tree, the tree 

 often takes two or more years to die, 

 but finally does so from the girdling. A 

 small amount of blight at the collar or 

 around the body does the maximum 

 amount of injury. 



This type of blight may be regarded 

 as the most injurious and more trees are 

 killed by this form as a rule than from 

 blight in the top. It is also the hardest 

 for the orchardist or inspector to find. 

 However, it produces one very striking 

 symptom that often leads to its detection. 

 The foliage on these girdled trees, or 

 even on the particular side of the tree 

 attacked, begins to take the reddish 

 autumnal color from midsummer on. In 

 walking through the orchard after the 

 first of August these reddish trees or 

 reddish parts of trees are usually dis- 

 tinguishable from a distance. These pre- 

 mature reddened or bronzed leaves often 

 contrast very strongly with the normal 

 dark green foliage. This is particularly 

 true of well-kept orchards. The red- 

 dening of the foliage is not always an in- 

 fallible guide as sometimes branches 

 color their leaves red from other diseases, 

 root troubles, girdling by mice, and un- 

 known causes. On examining these red- 

 dened trees, however, the blighted area 

 can usually be found. 



At least two types of frost injury also 

 very closely resemble pear blight at the 

 collar. In one of these the tree is injur- 

 ed from the soil line upward, usually on 

 the sunny side but not always so. Blight 

 is easily distinguished from this winter 

 sun scald when the latter occurs as an 

 elliptical are on the main trunk of the 

 tree from the soil or snow line nearly to 

 the branches. On the other hand, an- 

 other type of frost collar girdle reaches 

 but a little above the soil line or only oc- 

 casionally extends upward in a V-shaped 

 area but spreads well under the ground, 

 partially or wholly girdling the tree. 

 Still another type is that of root winter 

 killing. In this case the roots near the 

 surface of the ground are frozen and 

 killed, while the top of the tree may or 

 may not be injured by frost. It is neces- 

 sary to know these other collar injuries 

 in order to distinguish them from true 

 collar blight. 



(To he Continued in Next Issue) 



Photographs of pruning and spraying 

 scenes are requested for publication in 

 The Canadian Horticulturist. 



