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 
10°