SB 608 
 .T75 L5 
 Copy 1 
 
 PHOMA ROT OF TOMATOES 
 
 GEORGE K. K. LINK 
 
 and 
 
 F. C. MEIER 
 
 Pathologists 
 
 UNITED STATES DEPARTMENT OF AGRICULTURE 
 DEPARTMENT CIRCULAR 219 
 
 Contribution from the Rureau of Plant Industry 
 
 (OflBce of Cotton, Truck, and Forage Crop Disease Investigations) 
 
 WM. A. TAYLOR, Chief 
 
 Waahington, D. C. 
 
 May, 1922 
 
 tSHINQTON raOVERNMENTPRINTINQ OFFICE : Ittt 
 
 Monoevophk 
 

 PHOMA ROT OF TOMATOES 
 
 CONTENTS. 
 
 Page. 
 
 What is Phoma rot of the tomato? 
 
 What is the effect of Phoma rot on 
 the fruit? 
 
 What is the effect of Phoma blight 
 on the plant? 
 
 Where and when do the rot and 
 blight occur? 
 
 Where and how does the blight start 
 in the field? __ 
 
 Page. 
 
 Where and how does the fungus get 
 on the fruit? 
 
 What favors the start and develop- 
 ment of the fungus? 
 
 Does Phoma rot develop and spread 
 after the fruit is picked? 
 
 What should be done with the dis- 
 eased fruit? 
 
 How can the disease be controlled 
 in the field and in transit? 
 
 WHAT IS PHOMA ROT OF THE TOMATO? 
 
 PHOMA ROT of tomato fruits, also known as stem-rot, or black- 
 rot, is a disease caused by a mold, or fungus.^ This fungus also 
 attacks the leaves and stems of tomato vines, causing the disease 
 known as Phoma leaf -blight or stem-blight. 
 
 WHAT IS THE EFFECT OF PHOMA ROT ON THE FRUIT? 
 
 Phoma rot makes affected fruit unsalable, or at least lowers its 
 selling price, and often leads to its complete decay. 
 
 The fungus attacks both green and ripening fruit. In its earliest 
 stages, Phoma rot of green fruit is marked by minute, slightly sunken 
 spots. As these spots enlarge they develop brown or black borders 
 with slightly lighter centers which become dotted with black pimple- 
 like bodies (pycnidia) in which the seedlike bodies, or spores, of the 
 fungus are borne. 
 
 ^ This fungus is known as Phoma def<tructiv(i. A fungus (plural, fungi) is a small, 
 simple plant, which lacks the green color found in more complex plants, such as our 
 orchard, field, and garden crops. Molds, yeast, toadstools, and mushrooms are good 
 examples of fungi. Fungi can not make their own food, as green plants do, and many 
 of them obtain it from living plants or their dead remains. Those fungi which obtain 
 their food and energy from living plants are known as parasites, while the plants upon 
 which they grow and feed are known as their hosts. Such fungi usually cause harmful 
 changes in the structure, composition, and actiAities of their hosts, which are known as 
 lesions. Such changes constitute di.sease. Thus, in Phoma rot of tomato fruits the 
 fungus PJwma destrnctiva is the parasite which attacks and feeds upon its host, the 
 tomato, causing spots or lesions, which are manifestations of the disease. 
 
 4768—22 1 
 
2 Department Circular 219, U. S. Dept. of Agriculture. 
 
 On ripe fruit the earliest stages of the rot are marked by water- 
 soaked concave spots. As these enlarge, their advancing edge remains 
 slightly sunken, slightly discolored, and water-soaked. Their centers 
 become brown to charry black, leathery and firm, and dotted with 
 the black pimplelike fruiting bodies of the fungus. Under moist 
 conditions spores in the form of slimy whitish masses ooze out of these 
 fruiting bodies so abundantly that the fruiting bodies become com- 
 pletely covered and the center of the spot appears whitish and slimy. 
 The spots increase indefinitely and rapidly in size, but remain more or 
 less circular and concave. The underlying affected parts are brown 
 or black and remain firm until the spots become very large or infected 
 with organisms that cause soft rots. The dry crustlike center of. the 
 spot often cracks, and a grayish white growth of the fungus may 
 develop in the crevices. Frequently numerous small spots merge to 
 form a large one. 
 
 The spots occur most frequently at the stem end of the fruit, where 
 the fungus enters through growth cracks or breaks made when the 
 fruit is picked ; they are not confined to the stem end, however, since 
 the fungus may enter the fruit through breaks in the skin made by 
 insects, by cracking, or by other diseases, such as nailhead spot. 
 
 WHAT IS THE EFFECT OF PHOMA BLIGHT ON THE PLANT? 
 
 Phoma blight consists of spots on the leaves, leaf petioles, and the 
 stems of the vines. (PI. I.) Spotted leaves dry out and are shed 
 prematurely, thus causing a reduction of leaf surface, which lowers 
 the vigor of the plants and frequently causes heavy loss through 
 decrease in yield. The disease spreads to the fruit from these leaf 
 and stem spots, appearing on the fruit in the field and in transit as 
 the rot previously described. 
 
 In the early stages the spots on the leaves are mereiy small discol- 
 ored areas, later turning brown or black, which are definite in outline 
 and irregular in shape. These spots do not change color as they 
 become older, but do enlarge, and often several merge into a large 
 blotch of definite outline but irregular shape. They frequently start 
 along the leaf margins. On petioles and stems the diseased areas 
 appear as streaks or blotches. The dead areas on leaves, stems, and 
 petioles dry out and shrivel, and on them appear, singly or in groups, 
 the small black pimplelike fruiting bodies in which the spores of the 
 fungus are borne. 
 
 WHERE AND WHEN DO THE ROT AND BLIGHT OCCUR? 
 
 Phoma blight and rot occur most frecjuently in southern winter- 
 grown tomatoes, that is, in the crops of California, Cuba, Florida, 
 
Cir. 219, U. S. Dept. of Agriculture. 
 
 Plate I. 
 
 Phoma Rot of the Tomato. 
 
 The disease attacks tomato foliage as well as fruit. Fruits affected with growth cracks, 
 shown in the tomato just above the basket, are particularly susceptible. 
 
Phoma Rot of Tomatoes. 3 
 
 and Mexico. It has also been noted in Texas, Mississippi, Tennessee, 
 and in northern fields. 
 
 WHERE AND HOW DOES THE BLIGHT START IN THE FIELD? 
 
 The growing plant gets the disease from the soil where the fungus 
 may overwinter, possibly from seed extracted from tomatoes har- 
 vested in Phoma-infested fields, or possibly from near-by wild or 
 cultivated plants of the potato family infested with the fungus. 
 
 WHERE AND HOW DOES THE FUNGUS GET ON THE FRUIT? 
 
 As pointed out previously, the spores, or seedlike bodies of the 
 fungus, are produced in the black pimplelike bodies which appear 
 on the diseased spots of affected vines. These spores may be directly 
 carried to the fruit by rain, wind, insects, and man, or they may 
 become lodged in the soil and subsequently spattered on the fruit by 
 rain or carried to it by wind or insects. They probably are also car- 
 ried to fruit in picking and packing operations. 
 
 Fruit which carries these spores on its surface is said to be con- 
 taminated, and it is probable that most fruit picked in a diseased field 
 has spores on it, especially if the field is widely and severely affected. 
 
 A fruit may carry many spores of this fungus on its surface and 
 still be perfectly sound or healthy and remain so. Before the disease 
 can develop in a contaminated fruit, either in the field or after pick- 
 ing, infection must take place; that is, the spores must sprout, or 
 germinate, and the resulting fungus must enter the tissues of the 
 fruit and establish itself there. Laboratory and field studies indi- 
 cate that the fungus can not penetrate the uninjured skin of tomato 
 fruits; hence, only tomatoes the skin of which is broken by insects, 
 bruising, natural cracking, or other diseases can become infected. 
 For this reason Phoma rot occurs most frequently on the stem end of 
 fruits, either at growth cracks or in the fruit-stem scar. Wounds, 
 however, need not be so large as to be visible in order to permit the 
 entrance of this fungus. It is probable that infection of contami- 
 nated fruit is not restricted to the field, but that it also takes place 
 in packing houses and during transit. 
 
 WHAT FAVORS THE START AND DEVELOPMENT OF THE 
 
 FUNGUS? 
 
 The fungus can not grow if the temperature is either below 42° or 
 above 90° F., and its spores can not germinate if they are not sup- 
 plied with moisture. Consequently the disease can not develop and 
 spread in the field unless dews or rainy weather occur. The fungus 
 grows best when the temperature is about 83° F. 
 
4 Department Circular 219, U. S. Dept. of Agriculture. 
 
 As already stated, the fungus appaiiently can not penetrate the skin 
 of the tomato. As a consequence, contaminated fruit can not become 
 infected unless its skin is broken, even though temperature and 
 moisture conditions are favorable for the growth of the fungus. 
 
 Following infection some time must elapse before visible symptoms 
 of the disease appear. It may happen, therefore, that fruit is 
 packed which looks sound but which has become infected and will 
 develop visible signs of the disease later if temperature and moisture 
 conditions are favorable for the fungus. 
 
 Experiments show that at least 4 days must elapse before any signs 
 of the disease become visible in wounded contaminated fruit. After 
 10 days the spots become distinct, and after 3 weeks the black pimple- 
 like fruiting bodies appear. It was found that at 64° F. in a humid 
 atmosphere, spots may reach a diameter of 1 to 2 inches in 18 days. 
 The spots enlarge more rapidly in ripening than in green fruit. 
 For example, it was found that spots on ripe fruit were one-fifth to 
 three-fifths of an inch in diameter after 10 days, whereas the spots 
 on green fruit kept under the same conditions were only one-fifth as 
 large. 
 
 Under the most favorable conditions for the fungus, high hu- 
 midity and air temperature of about 80° F.. the disease becomes 
 visible on infected leaves in 3 to 5 days as slightly discolored spots. 
 In case of severe infection these spots give the leaves a speckled ap- 
 pearance. About 8 days after infection the spots become black and 
 are about one-tenth to two-fifths of an inch in size. 
 
 DOES PHOMA ROT DEVELOP AND SPREAD AFTER THE FRUIT 
 
 IS PICKED? 
 
 Every day's delay in getting infected fruit to the consumer 
 increases the possibility of loss. Fruit picked in an infected field 
 but appearing sound when wrapped, even though actually infected, 
 may show spots in 4 or 5 days. Furthermore, fruit which comes 
 from a disease-free field but which is packed or repacked in places 
 where diseased fruit or fruit from diseased fields is handled is likely 
 to become infected and develop the disease in transit or in the 
 ripening rooms. 
 
 WHAT SHOULD BE DONE WITH THE DISEASED FRUIT? 
 
 Spotted fruit, even though unsightly, is fit for food provided 
 the diseased parts are cut away. It should be disposed of as soon 
 as possible, however, to avoid greater loss, since the spots enlarge 
 rapidly and also open the way for other organisms of decay, which 
 may rot the fruit completely in a very short time. 
 
Phoma Rot of Tomatoes. ' 5 
 
 HOW CAN THE DISEASE BE CONTROLLED IN THE FIELD AND IN 
 
 TRANSIT? 
 
 No satisfactory effective measures for the control of the disease 
 in the field have been determined. Crop rotation and the planting 
 of disease-free seed probably are advisable. 
 
 Since the fungus can enter fruit only through wounds, it is highly 
 desirable that fruit be handled very carefully to avoid breaking the 
 skin. 
 
 The only effective way to prevent losses in transit or in the 
 marketing processes is to handle only fruit grown in disease-free 
 fields and not packed or repacked in places where Phoma-infected 
 fruit is handled. 
 
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