In chapter i it was noted that many species have been shown to be dependent for growth on one or other of the micro-nutrients, or at least have benefited by treatment with a micro-nutrient. It has been established that certain well-recognized pathological conditions met with in the field are associated with deficiency of a micro-nutrient, and some of these are so widespread or of such economic importance that they are designated by common names. Such, for example, are the grey speck disease of oats, and heart-rot of sugar beet. In this country diseases due to a deficiency of manganese and boron are both widespread and of economic importance; elsewhere shortages of zinc and copper have been shown to be responsible for diseases causing considerable damage to fruit crops. Well-defined diseases attributable to lack of molybdenum have also been recognized.

The more important of the deficiency diseases attributable to shortage of trace elements are described in this chapter.

DISEASES ATTRIBUTABLE TO A DEFICIENCY OF MANGANESE

The most general effect of manganese deficiency appears to be in the first place the development of small chlorotic patches localized in intervenal areas of the leaves. The form these patches take in different species is no doubt largely dependent on the anatomy of the leaf, so that in grasses with their parallel venation they tend to take an elongated form, producing ‘stripes’ or ’streaks', while in reticulate-veined dicotyledons they produce a spotted, speckled or mottled effect, as in potatoes and sugar beet. Other symptoms may follow, including reduction or cessation of growth and the development of necrotic areas which may not be limited to the affected regions of the leaf, but which may even affect the seeds, as in the case of the garden pea.

Grey speck of oats. The disease of oats most usually known as grey speck, but also sometimes called grey stripe, grey spot, or dry spot, is characterized by the appearance in the leaves of spots of a greyish colour, small chlorotic areas, chiefly in the lower half of the leaf, which tend to coalesce and form elongated streaks which finally turn brown. The first sign of the disease often occurs in young plants in the third or fourth leaf.