Seed Health Testing - Seed Testing - page 218

Figure 10.2. Corn plant infected by Spiroplasma kunkelii , a seedborne pathogen of corn (photo by Dr. Nicesio
F.J.A. Pinto, Embrapa, Brazil).
A signiicant advantage of ield inspection is that several diseases can be checked at one time.
Information on a disease also is available before harvest time, thus expediting the process of obtaining the
necessary oficial phytosanitary documentation to export the seeds. Field inspection, however, has some
serious limitations. It is a poor indicator of the plant pathogens that subsequently infect the seed. There are
few instances where the incidence of seed infection at harvest maturity has been shown to be related to level
of infection of a plant part such as stem or leaf. Block et al. (1999) showed that seed infection by Erwinia
stewartii could only be detected in corn ields in which over 50% of plant leaf tissue showed symptoms of
Stewart's wilt. This type of data is necessary for results of ield inspections to provide accurate estimates of
the incidence of seed infection. Other problems with the ield inspections are that ields are often examined
only once during the growing season. Pathogens that are symptomatic later in the growing season, such as
seed rots, or early in the season, such as seedling blights, will not be detected. Many diseases have masked
symptoms, and cannot be seen in the ield. Field inspection is also unsuitable for diseases such as bacterial
blight of soybean (Pseudomonas syringae pv glycines) which may be common in a region, is easily seen on
the plants, but cause little seed infection.