DISEASES OF PLANTS 79 



The first symptom of zinc deficiency was a decreased rate of 

 growth which began to be noticeable after about 3 months. 

 This was soon followed by the shoot apices acquiring a flat- 

 topped appearance due to the lower activity of the apical 

 meristem. The apical buds appeared bunched together while 

 secondary needles stopped growing and so appeared short, thick 

 and stiff and the bundles did not spread open. Chlorotic symp- 

 toms did not appear until the lapse of another 5 weeks. Then 

 small yellow dots appeared near the tip of the needles followed 



* 



by browning, soon giving the tops of the branches a bronzed 

 appearance. With the appearance of the first signs of chlorosis 

 in the leaves conical swellings sometimes arose on the root 

 apices. As far at least as the shoots are concerned the effect of 

 zinc deficiency on P. radiata thus resembles the effects produced 

 on other trees. 



It should be noted that the cultures grown by Smith and 

 Bayliss were non-mycorrhizal. 



White Bud of Maize. As stated early in this book (p. 3), 

 as long ago as 1914 Maze demonstrated the essentiality of zinc 

 for the growth of Zea mais. Maze reported that maize plants 

 grown in water culture deficient in zinc at first developed 

 normally, but that quite suddenly the leaves darkened and 

 developed a metallic sheen while nocturnal exudation became 

 very abundant, a deposit of soluble salts being left on the 

 leaves. Death of the plants followed in 3-5 days. 



According to the investigations of Barnette and Warner 

 (1935) it would appear that the disease of maize known in 

 Florida as white bud is due to zinc deficiency. They report the 

 disease as occurring chiefly on land which has been under 

 cultivation for a number of years and also on poorer land 

 recently brought into cultivation. The disease may sometimes 

 be serious enough for the crop to fail completely. 



The first symptoms of the disease may appear within a week 

 of the emergence of the seedlings from the soil. The first sign of 

 the trouble is the appearance of light yellow streaks between the 

 veins of the older leaves followed by the rapid development of 

 white necrotic spots. As the newer leaves unfold they are often 

 pale yellow to white in colour, a symptom which gives its name 

 to the disease. The older leaves develop light slate to dark brown 



