July 15, 1920 Relative Susceptibility to Citrus-Canker 349 



SUSCEPTIBILITY OF CITRUS FRUITS 



Citrus hystrix DC. (CPB 7872 and 2881, seedlings), I, II, III, in field, 1917 and 1918. 



"Cabayao" (CPB 7831, seedlings), I, II. 



Two types of these plants have been tested. One group has pointed leaves while 

 the second has rounded ends. Very little infection has been found on the plants 

 with the pointed leaves, either in the field or the greenhouse (PI. 61). However, 

 70 to 100 per cent of the leaves having rounded tips were infected with small to large 

 and scattering to many spots. Some defoliation occurred. Rather large spots of a 

 girdling type are common on thorns, twigs, branches, and old wood. 



Lee (j) finds that of the numbers tested by him in the Philippines seven were 

 severely infected, three moderately so, one slightly, while canker was not observed 

 on fotu-, and one proved resistant. As the group is obscure, although a large one, 

 some forms may be found resistant to canker. However, the majority, especially 

 those with rounded leaves, will prove to be almost as susceptible as grapefruit. 

 Citrus medica L. Citron of commerce (CPB 7768 and 7836, cuttings and seedlings), 



I, II, and III. "Sidro" (CPB 7816, seedlings), II. "Nana" (CPB 11281, seedlings), 



II, III. "Odorata" (CPB 11294, seedlings), II, III. "Etrog" (CPB 11178, seed- 

 ling), i.m. 



Of the citrons tested, the "Etrog" proved to be the most susceptible. All the 

 leaves were infected and some defoliation occmred. Twig and stem infections were 

 also present. A few twig and stem spots were found on CPB 7768, 7836, and 11281. 

 On the other numbers canker was limited to the foliage, the percentage of leaf infec- 

 tion varying from 30 to 100, with little defoliation. 



The spots were, for the most part, small and scattering and very distinct in charac- 

 ter. No doubt the texture of the leaves has a direct influence on the type of spot pro- 

 duced and also on the susceptibility of the leaves. The citrons, as a whole, while 

 rather easily infected, are not as susceptible as grapefruit but are more so than 

 Satsuma, {Citrus nohilis var. unshiu Swingle). 



Lee (j) tested 14 numbers in the Philippines and found i resistant, 5 on which 

 canker was not observed, 5 with medium infection, and 3 severely infected. He is of 

 the opinion that some of the citrons may be considered as canker-resistant. 

 Citrus sp. Small lemon (CPB 7833, seedlings), I,i II. Sweet lemon (CPB 1158, 



seedlings), I, II. "Davo lemon" (CPB 7837, seedlings), II, III. Limon real 18 



(CPB 7819, seedling), II. 



The plants of the lemon group so far tested have all proved more susceptible than 

 the citrons. All the plants in the experiments have few to many large twig and stem 

 spots, while 50 to 100 per cent of the leaves are infected. Canker also caused some 

 defoliation of the plants. 



Two t>-pes of spots are produced on the foliage. Where the texture of the leaf re- 

 sembles that of the citron the same kind of spot is produced, except that it is larger. 

 On the plants with smooth leaves the spots resemble those found on grapefruit (PI. 62). 

 In the scale of susceptibility, the lemons so far tested rank just below grapefruit. 



Lee's results (j) also show that the lemons are fairly susceptible under Philippine 

 conditions. 

 Citrus sp. Ichang lemon (CPB 11291 and 11204, seedlings), I, II, III, and I, 11,^ 



III, in field, 1918. 



The Ichang lemon was not considered under the lemon group because it appears to 

 be a natural hybrid, possibly betweeh lemon and pummelo. The plants are very 

 susceptible, for from 30 to 100 per cent of the leaves are infected and several plants 

 have severe twig and stem infections. All three plants in the isolation field were 

 reported infected during September, 1918, and August, 1919. However, all spots 

 were localized on the leaves. 



' Included in experiments of March 21, 1918. 



