TechNotes Oct. 10 - Grubs, Pythium on Poinsettias, Soil Temperatures

Grubs Looking Mighty Fine

Grubs are present throughout most of the United States. For most of the northern United States mid-October means that the grubs are finishing their feeding and beginning to migrate deeper in the soil to overwinter. This is a difficult time for turfgrass managers to decide whether to treat for grubs or not. In general, if grubs are present but no visible damage is occurring (ex. raccoon or skunk damage from foraging for grubs), treatment is not necessary. However, if grub damage is occurring to the turf because of their presence, treatment may be justified.

Controlling Pythium Root Rot on Poinsettia

Poinsettias are prone to Pythium root rots throughout the entire production cycle. The roots infected by Pythium are generally soft, mushy, and various shades of brown. The outer covering of the root, the cortex, is usually rotted and slides off easily when pulled, leaving the string-like vascular bundles behind. This is commonly referred to as sloughing of the roots.

Pythium usually attacks the root tips first and works its way upward in the root system. Wilting is the most recognizable symptom that poinsettias exhibit when they are being attacked by this pathogen.

Other above ground symptoms includes chlorosis and stunting. Pythium is an opportunistic pathogen; requiring damaged or stressed tissues to gain entry into the plant. Plants under stress caused by high soluble salt concentrations, poor drainage of the growing medium, and over-watering are particularly susceptible to Pythium infestations. Root injury from insects, such as feeding from fungus gnat larvae, can also provide an entry site for the pathogens through the feeding wounds.

Managing Pythium Root Rots

General recommendations for controlling root rot pathogens include using a porous, well-drained sterile potting medium, maintaining proper fertility levels, avoiding excess water, controlling fungus gnat and shore fly populations, and thoroughly cleaning and sanitizing the production area between crops.

The incidence of root rot pathogens can be reduced using proper water management. There is often a fine line between proper and improper irrigation practices. The longer a growing medium remains wet; there is a greater chance for Pythium to develop. Plants should be watered only when they need it and when conditions are conducive for it to be applied.

Always check the root system for root rots whenever there are signs of chlorosis, stunting, or wilting; this step could be the difference between saving the crop and losing it. Growers should remove the plant from the container and look for any signs of decay, brown-mushy roots, lesions, or discoloration. In many cases, these diseases can be detected early, and with the appropriate culture practices and chemical treatments, they can successfully be controlled.

Fungicides are often necessary to prevent Pythium root rot diseases from occurring or to control them, provided they have not reached epidemic proportions. Most growers apply fungicides as media drenches where the active ingredients can reach the infected roots. Fungicides are most effective when the pathogen levels are low and conditions are favorable for the disease to develop.

Several fungicides including products containing azoxystrobin, dimethomorph, etridiazole, mefenoxam, and propamocarb are effective at controlling Pythium on poinsettias. Many growers implement preventative fungicide programs, drenching poinsettias every 30 days using the active ingredients mentioned above. It is easier to control Pythium preventatively than it is to control existing populations that have already damaged plant roots.

--Paul Pilon, Perennial Solutions Consulting

Soil Temperature Map