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Plantwise Technical Factsheet

pod blight: soyabean (Diaporthe phaseolorum var. sojae)

Host plants / species affected
Abelmoschus esculentus (okra)
Abutilon theophrasti (velvet leaf)
Allium cepa (onion)
Allium sativum (garlic)
Amaranthus spinosus (spiny amaranth)
Arachis hypogaea (groundnut)
Capsicum frutescens (chilli)
Glycine max (soyabean)
Lupinus spp.
Phaseolus lunatus (lima bean)
Phaseolus vulgaris (common bean)
Solanum lycopersicum (tomato)
Vigna unguiculata (cowpea)
List of symptoms/signs
Fruit  -  abnormal shape
Fruit  -  mummification
Growing point  -  discoloration
Growing point  -  lesions
Leaves  -  necrotic areas
Leaves  -  yellowed or dead
Roots  -  cortex with lesions
Seeds  -  discolorations
Seeds  -  distortion
Seeds  -  mould
Seeds  -  shrivelled
Stems  -  discoloration
Stems  -  internal discoloration
Whole plant  -  discoloration
Whole plant  -  early senescence
Whole plant  -  plant dead; dieback
Whole plant  -  wilt
D. phaseolorum var. sojae produces light-brown spots on the cotyledons and lower stem. Pod and stem blight later appear as pycnidia on the main stem, and broken upper and lower branches, petioles, leaves and pods after maturity (Athow and Laviolette, 1973; Dimitrijevic and Jurkovic, 1982). The most important aspect of pod and stem blight is its effect on seed. Infected seeds may exhibit varying degrees of cracking on the seed coat and shrivelling, and are frequently covered with white mould (Athow and Laviolette, 1973). Sometimes the seeds have brown or black spots on the seed coat (Ilyas et al., 1975). Seeds infected with D. phaseolorum var. sojae are frequently flattened, wrinkled, discoloured and smaller than non-infected seeds (Ellis et al., 1974a).
Prevention and control
Cultural Control

The diseases caused by the Diaporthe/Phomopsis complex can be partially controlled by cultural practices such as clean tillage and crop rotation with non-host crops. Knowledge of the inoculum sources is basic for these control strategies (Athow, 1987).

In Brazil, the incidence of D. phaseolorum var. sojae on seeds increased with delayed harvesting (Dhingra et al., 1979a). Heavy rainfalls during August in central Illinois, USA, favoured an epidemic of soyabean pod and stem blight. Even under these severe weather conditions, the quality of seeds harvested from the upper portions of soyabean plants was not reduced. Selective harvest would therefore be feasible and may avoid seed quality losses (Hepperly and Sinclair, 1980a).

Delayed planting decreased infection by D. phaseolorum var. sojae and increased germination rates. Late season cultivars showed less infection and greater germination of seeds than early season cultivars. As plant density increased, infection by D. phaseolorum var. sojae increased and germination decreased. Seeds treated with benomyl germinated better at 15°C than at 25°C. An increased level of N induced greater infection by D. phaseolorum var. sojae and lower germination. A delay in harvest caused a reduction of seed germination and increased the occurrence of abnormal germination and seed decay. This trend was more conspicuous in seeds from soyabean plants that had not been treated with benomyl than in those that had been sprayed with the fungicide (Chin et al., 1993a). Infection of seed by D. phaseolorum var. sojae, Glomerella glycines and Fusarium spp. was lowest, and germination percentages highest, on late-maturing cultivars that were harvested early (Alexander and Hinson, 1973). However, the physiological quality and health of seeds were decreased in Goias, Brazil, by early sowing due to adverse moisture conditions, mechanical damage at harvesting and fungal infection, mainly by D. phaseolorum var. sojae (Pereira et al., 2000).

Field sanitation, including the removal of host debris, fallen petioles and cotyledons from the field, reduced the infection of pods and seeds by Phomopsis spp.; however, seed infection was 28.7% in the sanitized field. Fields sanitized by the application of benomyl around the soyabean plant decreased seed infection by Phomopsis spp. Total seed infection caused by various pathogens was 75-79% without benomyl application compared to 34-42% with a routine fungicide application schedule. Field sanitation was effective in controlling Phomopsis seed decay when infection pressure was low. However, it did not significantly increase the yield of soyabean, whereas a routine fungicide application schedule did (Oh Jeung Haing, 1998).

Chemical Control

Due to the variable regulations around (de-)registration of pesticides, we are for the moment not including any specific chemical control recommendations. For further information, we recommend you visit the following resources:

Host-Plant Resistance

Nearly 1000 breeding lines and 35 cultivars were scored in the field for resistance to several diseases. None of the 35 cultivars occurred in the top 100 entries for resistance to D. phaseolorum var. sojae. In harvested seed from the 21 most resistant lines, the incidence of D. phaseolorum var. sojae and other pathogens was 0-5%, whereas in >800 susceptible lines the incidence of infection was 100% and many seeds were severely shrivelled. Eleven resistant lines had a common lineage to PI227687 and PI229358. Two other D. phaseolorum var. sojae resistant lines were descended from the Brazilian cv. Santa Maria. These resistant lines also had resistance to several insects and microorganisms including Meloidogyne, C. kikuchii, Peronospora, C. truncatum, C. sojina, Xanthomonas campestris pv. glycines and Soyabean mosaic virus (Berger and Hinson, 1984).

The level of infection may be related to cellular appendix and structural barriers. Lesions on green pods were centred about trichomes. Sparsely pubescent cv. Chippewa developed fewer pod lesions than the pubescent cultivars Corsoy, Hark and Harosoy. Cultivars with impermeable seed coats had significantly less infection than those with permeable seed coats. The results suggested that cv. Sooty, with a black, impermeable seed coat, showed some genetic resistance to Phomopsis infection (Yaklich and Kulik, 1987).

Disease assessment of inoculated intact plants and detached pods of cultivars Hark, Rampage, Wells and Williams showed a close correlation between the two methods. Cultivars Hark and Williams were less susceptible to D. phaseolorum var. sojae than cultivars Rampage and Wells (Hepperly and Sinclair, 1980b).

The extent of internal seedborne infection by D. phaseolorum var. sojae is influenced by varietal differences and weather conditions when the plants are nearing maturity (Ellis et al., 1974a; Hepperly and Sinclair, 1980a). Plants of cv. Hark showed relative resistance to pod and stem blight, with few symptoms and little seed damage, whereas the cultivars Chippewa 64 and Wells were more susceptible to the disease. Not all lines were predisposed to seed infection by D. phaseolorum var. sojae after inoculation with Soyabean mosaic virus (SMV). Midwest, PI360.835, PI86.146 and PI181.549 were resistant to D. phaseolorum var. sojae whether inoculated with SMV or not. Williams was resistant if not inoculated with SMV but susceptible if inoculated with the virus. Varieties which appeared equally resistant when not inoculated with SMV showed differences in susceptibility when inoculated. Hence, differences may exist among lines in the degree or mechanism of resistance. Lines not predisposed by SMV to infection with D. phaseolorum var. sojae are preferable when breeding for improved seed quality (Hepperly et al., 1979). In Brazil, cultivars UFV-2, UFV-72-4 and UFV-72-3, with 0-3% D. phaseolorum var. sojae, 2-10% other fungi, and 84-91% germination, are recommended for seed production in the Vicosa region of Minas Gerais (Dhingra et al., 1979a). The use of resistant cultivars is also recommended in Cameroon (Bernaux, 1979).

Moderately resistant cultivars may be able to modify their behaviour with age. Pods from four soyabean cultivars were aged for 1 week and then dried (accelerated ageing test). Seeds from aged pods had lower scores in vigour tests and were more susceptible to infection by Phomopsis phaseoli than non-aged seeds (Yaklich and Kulik, 1987). Soyabean cv. Coles (susceptible) and breeding line OX615 (moderately resistant) were evaluated in field trials over 3 years for the time and percentage of pod and seed infection caused by Phomopsis longicolla, D. phaseolorum var. caulivora and D. phaseolorum var. sojae. Pod infection increased linearly with time; OX615 showed, on average, 15% less infection than cv. Coles at each sampling date. Meanwhile, seed infection increased curvilinearly in conjunction with decreasing seed moisture, with marked increases between the fifth and seventh weeks after flowering. At maturity, average seed infection was 69% in cv. Coles and 28% in OX615 (Anderson et al., 1995).

P. longicolla and D. phaseolorum var. sojae are the principal causal organisms of Phomopsis seed decay (PSD). No commercial cultivars have been found to be resistant to PSD. However, the plant introduction PI417479 constituted an important source of genetic resistance. When grown under field conditions favourable to infection by Phomopsis spp., PI417479 was free of seed infection in two tests and had 3% infection in another. In the same environments, cv. Williams 82 had 25-59% infection. Inheritance of the trait was determined to provide information for the efficient transfer of the resistance to improved cultivars. Crosses were made between PI417479 and two susceptible genotypes. Five generations were developed for each cross and tested at two locations. Plots were artificially inoculated to enhance infection. Seeds from plants showing varying degrees of infection in the first season were progeny tested. The environment strongly influenced disease incidence, but results indicated that two complementary dominant nuclear genes controlled resistance to PSD. It was concluded that these studies will facilitate the development of resistant cultivars (Minor et al., 1995).
Human consumption of cooked soyabeans and soy flour may help to alleviate a global deficiency in protein, especially in highly populated developing countries (Circle and Smith, 1975). Soyabean seed quality, determined by germination and the general appearance of the seed, is therefore of great importance in many soyabean-growing areas of the world. Tropical areas have high rainfall and high temperatures, which favour the development of seedborne D. phaseolorum var. sojae. The pathogen is generally considered a weak parasite, but its wide distribution and high frequency suggest that it deserves careful attention.

Pod and stem blight has frequently been cited as a common disease in Illinois, Iowa and Indiana in the USA and Ontario in Canada (Athow and Caldwell, 1954; Gerdemann, 1954; Dunleavy, 1956; Hildebrand, 1956). Later, in the midwestern states of Maryland and Delaware, D. phaseolorum var. sojae was considered to be the predominant organism associated with low seed quality (Athow and Laviolette, 1973). Seed lots from Louisiana, bioassayed for internally-borne pathogens, showed 97% infection by D. phaseolorum var. sojae (Ellis et al., 1974a).

Pods infected with D. phaseolorum var. sojae are unsuitable for vegetable use (Mehlembacher et al., 1977) and soyabean oil must be colourless for acceptance in food products (Bailey, 1964). In Illinois, soyabean seeds showing symptoms of D. phaseolorum var. sojae infection were smaller in size and volume, lower in density, produced lower quality oil and flour, and had lower viability and durability than symptomless seeds. Oil from infected seeds had a rancid, off-odour and a high peroxide value indicating oil deterioration (Hepperly and Sinclair, 1978).

Pod and stem blight, Phomopsis seed decay and stem canker are the diseases that most severely affect soyabean seed quality and yield. These diseases are present in almost every soyabean-producing region of the world (Athow, 1987).

In Europe, D. phaseolorum var. sojae was found for the first time in Romania in 1972 (Hulea et al., 1973), then in Hungary in 1975 (Ersek, 1978; Kovics, 1982) and later in the Serbo-Croatian region (Dimitrijevic and Jurkovic, 1982; Vrataric et al., 1991). It was isolated from soyabean plants and seeds in Senegal and Cameroon in the 1970s (Girard, 1979; Bernaux, 1979).

In South America, D. phaseolorum var. sojae was identified on naturally infected soyabean plants in Argentina (Anon., 1979) and Brazil (Castro and Kimati, 1981; Almeida, 1981; Berger and Hinson, 1984) and pathogenicity was confirmed in Venezuela (Sanabria de Albarracin, 1993). Other studies have reported the importance of Phomopsis spp. (D. phaseolorum var. sojae, P. longicolla, Phomopsis spp.) as seedborne pathogens associated with green seed soyabean (eda mamé) (Pioli et al., 1997). Pioli et al. (2000) discuss the epidemiological role of Phomopsis spp. through the progress of infection and the tissues colonized by the fungus in core soyabean-growing areas (Pioli et al., 2000).
Related treatment support
External factsheets
Crop Science Extension & Outreach Factsheets, College of ACES, University of Illinois, Urbana Champaign, USA, English language
UF/IFAS Factsheets, University of Florida, 2001, English language
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