PART 11. L
diseases by inbreeding them for many generations, ther attenuated
but almost homozygous plants would then be double crossed in the
usual way, keeping resistance to disease and regaining vigour owing to
a recombination of factors for yield. Fortunately, though there have
been rust scares, there seems little danger of maize being destroyed
by any fell disease in this Colony. That is one of the advantages ol
maize. As long as the cut worm and stalk-borer are defeated, and
the rain holds out, there is little fear of the maize crop not coming
to fruition.
As regards wheat, it cannot be too strongly stressed that the low
yields are largely due to growing the crop on soils which are not
wheat soils. It is to be hoped that this will be realised as time goes
on, and that wheat will in future be confined to wheat land.
The other cause of low yields is rust. Some idea of the ravages
of rust in this country will be realised when it is stated that year after
year whole fields of wheat are completely destroyed by this disease.
No wonder only 20,000 acres are under wheat and that after all these
years this country is not yet supplying all its own requirements in
flour. Heretofore a farmer has taken a grave risk in growing wheat,
and yet it appears to be the only crop of its kind to grow at high
altitudes, and at low altitudes with a short rainfall.
_ Some accounts must now be given of rust. - There are three
different species of rust in this country. These three species are
Puccinia graminis, Puccinia glumarum, and - Puccinia triticina.
Either of the first two will alone completely destroy a crop of wheat.
Puccima triticina, though it does affect the yield, is not so serious in
this country as the other two. Puccinia graminis is parasitic on the
chaff, stem, leaves, and leaf sheaths of the wheat plant; Puccinia
glumarum on the leaves, leaf sheaths and chaff; Puccinia triticina on
the leaves and leaf sheaths. Not only are there three species of rust,
but there are many biologic forms of each species. In the case of
Puccinia graminis forty different forms have so far been isolated.
These biologic forms cannot be differentiated microscopically,
they can only be differentiated by the reactions of certain varieties of
wheat to inoculation by them. Some varieties are resistant to one
or more and susceptible to others. Furthermore there are several
degrees of resistance ranging from immunity to almost complete
susceptibility.
There are two kinds of resistance to rust; these are physiological
and morphological.
| Physiological resistance will be described first. A spore is blown
on to the wheat plant, there it germinates and forms an appresoriuia
over a stomutal opening. The germ tube then enters the stomata
and hyphae begin to grow between the ceils of the plant. Immediately
the cells of the wheat plant are touched by the hyphae th-y die.
The consequence is that the mycelium is unable to fini food and as
soon as the food in the mother spore is exhausted the rust plent then
dies. Tt is not able to mature and form pustules and a'! that is seen
on the wheat plant is a small yellowish white fleck which consists of
dead cells. If these white flecks are seen on a variety of wheat it
is likely that the variety is immune to the biologic forms of
rust which have attacked it. In the case of lesser deorees of
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