Value of phloem necrosis in the diagnosis of potato leaf‐roll

Abstract

In plants infected with leaf‐roll virus a type of phloem obliteration and necrosis occurs which is distinct from any abnormality produced by other pathogens or arising from physiological causes. The necrosis occurs in the primary phloem only of the bicollateral bundles. The affected tissue reacts with phloro‐glucinol in HCI. It was present in all of 179 plants of 33 varieties showing secondary leaf‐roll which were examined and was not found in any of 83 healthy plants of 20 varieties. The amount of necrosis varies in different plants. If the disease is severe, necrosis may extend to almost all parts of the plant except the stolon, tubers and roots. If the infection is mild it may be confined to a very few strands in two or three nodes near the base of the main stem. Phloem necrosis can always be found before leaf rolling is apparent. In primary leaf‐roll, slight necrosis can be found in the stem near the bases of the lowest rolled leaves and sometimes in the petioles. A technique is suggested for the use of this symptom in diagnosis.

The Significance of the Necrotic Phloem Reaction in the Potato to The Leaf-Roll Virus

“…Table 5 shows clearly that in plants with well-developed phloem necrosis there is a gradation in the amount of necrosis from the base to the tip, the occurrence of necrosis of the internal primary phloem following much the same pattern. These results are in agreement with Sheffield's (1943) observation that phloem necrosis is usually at a maximum in the basal nodes of infected plants. There were a few cases among the hybrids of plants with a higher percentage of necrosis in mid-stem sections than in basal sections.…”

“…It is apparent from the results shown in Table 2 that under the condi- tions of this experiment sensitivity 'of the primary phloem, as measured by the extent of necrosis, is a varietal factor. These results provide concrete evidence for Sheffield's (1943) suggestion that varietal differences in the amount of phloem necrosis may occur. Plate 1, Figure 1, is a photomicrograph of a section from the base of the stem of a leaf-roIl-infected Katahdin plant showing the stained areas in the external primary phloem.…”

“…The nature and occurrence of phloem necrosis in leaf-rolf-affected potato plants have been described by Artschwager (1923), Quanjer (1931), Bawden ( 1932), and Sheffield (1943). The reaction is usually well defined and quite distinct from top necrosis (Bawden 1932), virus Y (Hutton 1948) and potato rosette (Hutton and Oldaker 1949).…”

“…The lack of a qu!ck arid accurate method of leaf-roll detection has retarded leafroll research for some time. It was found that the technique developed by Sheffield (1943) and improved by Wilson (1948) for the diagnosis of leaf roll from the occurrence of necrotic areas in the external or internal primary phloem of basal stem sections was a valuable aid in leaf-roll experiments. The main .disadvantages of this improved technique were the irritation to the operator and the corrosion to the microscope resulting from hydrochloric acid fumes.…”

Some Factors Affecting Localized and Systemic Necrotic Reactions to Virus Y in the Potato

“…It was made possible by using a modification of the method used by Sheffield (1943) in the diagnosis of potato leaf roll. Wilson (1948) has pointed out that phloem necrosis in leaf-roll affected potato plants is more easily distinguished if concentrated hydrochloric acid is used in Sheffield's method.…”