A complex of soybean diseases is caused by species from the genus Diaporthe
/Phomopsis. Diaporthe phaseolorum (anamorph Phomopsis phaseoli) and Phomopsis
longicolla (teleomorph unknown) are described as soybean pathogens. The first
species includes three varieties: D. phaseolorum var. sojae, anamorph
Phomopsis sojae, the causal agent of pod and stem blight, and D. phaseolorum
var. caulivora and D. phaseolorum var. meridionalis, agents of northern and
southern stem canker. The species P. longicolla is the most common and most
damaging agent of soybean seed decay. The diseases caused by parasites from
this genus were first observed and described on soybean in the USA. Presently
they are widespread in most soybean-growing regions around the world. Soybean
in Serbia is attacked by all pathogens mentioned above, except for D.
phaseolorum var. meridionalis, the causal agent of the southern stem canker.
D. phaseolorum var. caulivora (northern stem canker) has the greatest
economic importance because it causes wilt and drying of plants during pod
development and grain filling. Most intensive outbreaks of the disease
occurred in the 1980s in southern and southeastern Banat, southern Backa and
Srem. Prematurely wilted plants yielded 50% to 62% (depending on variety)
less than healthy plants. Such heavy losses raised the question of the
profitability of soybean growing. P. sojae and P. longicolla are less
important in Serbia at the moment. Intensive infections of soybean seed break
out occasionally. It was found that P. longicolla is the primary agent of
seed decay and latent infections of seed in our country, although the other
members of this genus may cause identical symptoms. Sexual cycle of
development (teleomorph stage) of the fungi of the genus Diaporthe /Phomopsis
form perithecia with asci and ascospores, while in the asexual cycle
(anamorph stage) various types of conidia (? and ?) are formed in pycnidia.
The species P. longicolla has no teleomorph stage. Infected harvest residues
and soybean seeds are main sources of the inoculum. D. phaseolorum var.
caulivora forms perithecia with ascospores on overwintered harvest residues.
During growing season, ascospores infect leaf laminas, petioles, or injured
plant parts. P. sojae and P. longicolla form pycnidia (P. sojae sometimes
forms perithecia), and they infect plants by means of conidia. Seed infection
by conidia or ascospores unfolds via pods. Infected crop residues are the
main source of the inoculum, while infected seeds help the parasites spread
over large distances. Humidity and temperature (soil and air) are the mains
factors that affect the dynamics of fruiting body formation, spore release,
establishment of infection and the development of disease symptoms on soybean
plants. Considerable attention is devoted to finding effective sources of
resistance to the disease complex caused by species from the genus
Diaporthe/Phomopsis. Resistance to the agents of seed decay was found in
several introduced (PI) genotypes. This resistance is controlled by one or
two pairs of dominant genes. Some cultivars are resistant to D. phaseolorum
var. meridionalis (southern stem canker). Their resistance is controlled by
two or four pairs of genes (Rdc). There is no physiological resistance to D.
phaseolorum var. caulivora, the agent of northern stem canker, but
significant differences in susceptibility level exist among soybean
genotypes. Molecular techniques (RAPD, PCR-RFLP) showed that significant
variability was present within the population of D. phaseolorum var. sojae.
Other species of fungi were suspected to exist within that population, which
could not be distinguished on the basis of their morphological
characteristics. This was subsequently confirmed by the sequencing of
different regions of ribosomal DNA.
Causal Agent of Red Stripe Disease of Rice
