ABSTRACT

Diseases that can detrimentally affect soybean yield and seed quality range from seed rot and ‘damping-off’ of seedlings to foliar blights and seed infections (Hartman et al. 2015c). Susceptible soybean plants are vulnerable to infection by viruses, bacteria, mollicutes (bacteria that lack cell walls), oomycetes and fungi, and can be parasitized by nematodes from several genera. Some of the most damaging diseases of soybean grown in North America are listed in Table 1. Wrather and Koenning (2009) estimated that the four most economically important soybean diseases and pests in the United States between 1996 and 2007, in order of importance, were soybean cyst nematode (SCN; Heterodera glycines); Phytophthora root and stem rot (PRR; Phytophthora sojae); seedling diseases caused by Fusarium spp., Pythium spp. and Rhizoctonia solani; and charcoal rot (Macrophomina phaseolina). SCN and PRR are routinely listed as the first and second most economically important diseases of soybean grown in North America (Koenning and Wrather 2010). This is partly because they are soil pathogens, so once established in a

Ta b

le 1

S o

m e

o ft

he m

aj o

r d

is ea

se s

o fs

o yb

ea n

in t

he U

ni te

d S

ta te

s an

d C

an ad

a an

d t

he r

el at

iv e

im p

o rt

an ce

o fr

es is

ta nc

e in

n o

rt he

rn a

nd s

o ut

he rn

p

ro d

uc tio

n re

g io

ns

field, they will remain a threat in subsequent years and cannot be eradicated. In addition, PRR is primarily a problem in the major production regions of the northern United States and adjacent regions in Canada. Because of the destructive potential of these diseases and their widespread distribution in both northern and southern soybean-producing areas in the United States, few modern cultivars lack resistance to SCN or PRR. In contrast, losses attributed to some other diseases can vary considerably from one growing season to another, largely due to differences in precipitation and temperatures. For example, white mould, or Sclerotinia stem rot (Sclerotinia sclerotiorum) (SSR), was ranked as the second most destructive disease in the northern United States in 2004, but caused only negligible losses in the southern states that year, and had not even been among the top ten disease problems in the northern states in 2003 (Wrather and Koenning 2006).