This type of resistance is effective against all races of a pathogen that may exist in any given field. However, it is not complete resistance. Some disease does develop, but it is limited when compared to a susceptible variety. This type of resistance is typically controlled by several genes and is subsequently more difficult to incorporate into commercial varieties. Both Phytophthora and Sclerotinia can be managed with this type of resistance. Partial resistance has also been called field resistance or tolerance.
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| Figure 3. There are fewer roots colonized and at a reduced rate in soybean varieties with partial resistance to Phytophthora sojae. The roots are taken from pots where the soybeans were forced to grow through a 1/4-inch layer of P. sojae inoculum. The partial resistance scores for these varieties, from left to right, are control, 3.5, 4.5, and 6.0 on a scale of 1 to 9 where 1 = no visible root rot and 9 = all plants dead. |
Soybean plants with partial resistance to Phytophthora have fewer lesions on the roots and reduced colonization of the roots. Soybean varieties with partial resistance to Sclerotinia have fewer numbers of plants that develop stem rot, and the level of disease is reduced overall. Environmental conditions and the amount of the pathogen present in fields can limit the effectiveness of this resistance. If the environmental conditions are highly favorable for disease development and/or the pathogen is present in very high numbers, then partial resistance can be overwhelmed and disease incidence may reach economic thresholds.
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