Cabbage Integrated Pest Management : An Ecological Guide.

Major Diseases of CabbageþHow to test your soil for presence of clubroot:The easiest way is to uproot several cabbage roots at harvest and check them for clubs. If you findsome roots with clubs, the soil is infected.When you do not have that possibility, another option to check the soil for presence of clubroot is thefollowing trial. This trial is however more time consuming (takes at least one month) and there is arisk that you will find a “false negative”: the soil is infected but you don’t find it in the trial.Take some soil from different portions of the field. Put it in pots and sow some cabbage seed (canalso be Chinese cabbage as this is very sensitive to clubroot) per pot. Keep the soil in the pots moistand look after the emerging seedlings for about 3 - 4 weeks. Seedlings with at least 3 true leaves canbe uprooted, and washed to remove the sand from the roots. Carefully check the roots for smallclubs. If you find clubs, the soil is infected.Since clubroot can be very localized in a field, the more soil samples and pots you take for this test,the better your conclusion will be. That reduces the risk of missing an infection.Role of environmental factorsThe temperature range in which the fungus is active is 9 - 35 o C with an optimum temperature for fungaldevelopment of 24 o C. Infection is limited by low soil moisture. A common observation is that the diseaseis most severe in low-lying, poorly drained soils.A soil pH of less than 7 favors disease development. When the pH is 7 or higher, the spores of the fungusgerminate poorly or not at all.W here potatoes are grown in rotation with cabbage:While clubroot of crucifers is suppressed by a soil pH above 7, potato scab (Rhizoctonia solani) isknown to be stimulated when the soil pH raises. This is particularly true in sandy soils, less in claysoils. A pH below 5.2 will reduce soil-borne scab in potato. That means that when soil pH is raisedwith lime to levels above 5.2, chances are that a following potato crop may develop symptoms ofscab. In general, potatoes prefer a more acidic soil than cabbages. It depends on factors like severityof the diseases, economics, etc. what crop should have the priority of soil pH adjustment.Importance - physiological impact - plant compensationGiven the fact that once the soil is infected with clubroot, the disease can stay in the soil for more than7 years, even if no cabbages or other cruciferous crops are grown, the fungus is one of the most persistentdiseases known in vegetable growing. In an infected field however, only seldom all plants are affected.Mostly the disease is localized, with groups of plants dropping out. Therefore, in an infected field, it is stillpossible to grow cabbage. The key is to keep the fungus from multiplying too much. See under preventionactivities below. Severely infected fields however, become unfit for cabbage (and other crucifer) cultivation.Once the fungus enters the fine roots of a cabbage plant, the plant reacts by making the clubs, cancerlikeenlargements in the roots. Water and nutrient uptake are severely restricted by this and the plantcannot develop normally but it will not die immediately because there are still some roots left for waterand nutrient uptake. In the middle of a warm, sunny day however, the plant may wilt.Usually, symptoms are localized near the point of infection. The fungus does not move through the plantveins. When only a small part of the root system is infected with clubroot, a head can still be formed.New roots are generated that will compensate for the loss of part of the roots. Often, these plants and theheads will be small in size but still look relatively healthy. When a plant is suspected of having clubrootCabbage Ecological Guide - 2000164