Agriculture Reference
In-Depth Information
direct Visual Examination
Seeds may be assayed by direct visual examination of the pathogen as it appears in, on, or accompanying
the seed. The pathogen is not altered in any way, but seed itself may be altered to facilitate detection of the
pathogen. Three general methods for direct visual examination exist.
Figure 10.3. Left: Soybean seed with a purple stain on the seed coat caused by
Cercospora kikuchii
; Right:
Sclerotinia sclerotiorum
accompanying a soybean seed lot.
Visual examination of seeds.
Seeds are examined with, or without microscopes, for signs or symp-
toms of the pathogen either on the seed, or as accompanying structures. Symptoms could include physical
changes, including color, size, or shape. Accompanying structures could include nematode galls, sclerotia,
ergot, or soil particles (Fig. 10.3).
An advantage of this protocol is that the test is usually very quick and no special equipment is required
except for a stereoscopic microscope. Limitations are that the tests often have poor sensitivity, detecting
only severely infected seeds. Furthermore, few pathogens express clear enough signs or symptoms to allow
for accurate identiication.
Visual examination of internal tissues.
Seeds usually are altered to allow the pathogen to be sepa-
rated or to facilitate staining of the pathogen within seed tissues. Fungal mycelium or spores are then exam-
ined under a microscope. Loose smut pathogens of barley
(Ustilago nuda)
and wheat
(U. tritici)
are tested
by digesting seeds in sodium hydroxide. Embryos are separated from seed chaff in a iltration procedure.
Embryos are then cleared in lactophenol and examined under a stereoscopic microscope for the presence
of mycelium (Fig. 10.4)
. Gloeotinia temulenta,
the cause of blind seed disease is detected in ryegrass seeds
by soaking seeds in water droplets, spreading the glumes apart; conidia emerge in a milky exudate and are
easily observed under a microscope.
These procedures usually have high sensitivity because very small amounts of the pathogen are detect-
able and individual seeds usually are tested. Limitations of the procedures are that few pathogens can be
tested in this way and the test is usually labor intensive.
