How to Manage Pests
UC Pest Management Guidelines
Pathogens: Pythium ultimum, P. aphanidermatum, Rhizoctonia solani, Aphanomyces cochlioides
(Reviewed 11/05, updated 1/10)
In this Guideline:
SYMPTOMS AND SIGNS
Seedling diseases can appear as seed decays, preemergence damping-off, or postemergence damping-off. Depending on the pathogen, most of the seed tissue is susceptible to infection, including nongerminated seed, germinating radicle, and emerging seedling up through the four- to six-leaf stage. Preemergence damping-off appears as darkened lesions on the emerging radicle and causes death of the radicle and seedling. Postemergence damping-off appears as a lesion on the seedling root or crown tissue, and causes the seedling to wilt, and possibly die. Plants that survive infection will not grow vigorously, resulting in greatly reduced yields.
COMMENTS ON THE DISEASE
The four pathogens that cause seedling diseases of sugarbeet are soilborne. Pythium ultimum is widespread in soil and attacks many crops. It infects unprotected seedlings at temperatures favorable for germination of beet seed (75° to 86°F), especially in winter and spring under conditions of warming soils with a high moisture content. It primarily causes a preemergence damping-off, but under moist conditions a postemergence damping-off may occur. Pythium aphanidermatum attacks seedlings only in warm soils (86° to 95°F, 30° to 35°C) with abundant soil moisture. Rhizoctonia solani and Aphanomyces spp. are problems primarily on emerged seedlings when temperatures are above 68° to 86°F.
To minimize the potential for seedling diseases, use methods that favor rapid seedling emergence, including planting seeds as shallowly as practical and managing soil moisture (preplant irrigate, seed into moist soil and delay second irrigation until seedlings are beyond susceptible stages). Where Rhizoctonia is a problem, avoid planting beets following beans and other legumes, or cotton.
Buy seeds treated with protective fungicides that are effective against the pathogens in the soil to be planted. Seed treated with chloroneb has protection against Rhizoctonia solani. Mefenoxam-treated seed protects against Pythium. Currently, there are no registered fungicides in California that provide effective protection against Aphanomyces spp. In fields where Aphanomyces spp. are present, follow practices that enhance rapid germination, plant when the weather is cool, avoid saturated soil conditions in the seedbed, and rotate the crop with nonhost crops.
UC IPM Pest Management Guidelines:
S. Kaffka, Agronomy and Range Science, UC Davis
Acknowledgement for contributions to Diseases:R. T. Lewellen, USDA, Salinas
C. A. Frate, UC Cooperative Extension, Tulare County