Yoel Messika – Manager, Agro-technique and Guidance Unit

yoel.messika@zeraim.com

14/09/2009

 

As summer approaches we see an increase in the number of pests such as leaf miners, mites (Red spider mitesand Tomato russet mites), whitefly and various, caterpillars. Shadehouses planted in the spring, with developed foliage are liable to be attacked by leaf and soil diseases (Bacterial wilt and Verticillium Wilt in tomatoes). At the same time it is important to continue preventative treatment against leaf diseases by using one of the preventative solutions such as sulfur compounds for powdery mildew diseases and manganese compounds to prevent the development of Late blightearly blightand leaf mold.

 

This disease is very active during the spring/summer season. It first develops (primarily) in the lower older leaves and later rises to the upper leaves. In addition, in recent years, more and more tomato plants have been found to be infected with Erysiphe spp as opposed to Leveillula taurica (which appears as yellow spots on the upper side of the leaves while on the under side it appears as white spots which hold the fungus spores).  With Erysiphe spp white spots (fungus mycelium and spores) appear on the upper side of the leaves. It is imperative to prevent the development of the disease by removing infected leaves when infection is low and spraying regularly once a week with various groups of fungicides, switching between fungicides from different groups in order to prevent the development of resistance and to retain the effectiveness of existing substances. 

The various groups include liquid sulfur-based substances which efficiently eliminate also,russet mite;the triazol group (penconazole,difenconazole,triadimenol,bromuconazole,cyproconazole, ,tebuconazole,myclobutanil); the strobilurin group (azoxystrobin trifloxystrobin) and the new designated groups (cyflufenamid,boscalid+pyraclostrobin, ,bacillus subtilis neem oil).

 

This disease primarily appears on the older leaves of the plant or on leaves with severe leaf miner infestation where the disease develops on the lesions. In such cases, effective treatment against leaf miners and against the disease is recommended. Treatment may be effective with substances such as:, difenconazole, tebuconazole, bromuconazole Mancozeb products,chlorothlonil.

 

This disease attacks the entire tomato plant as it breaks down resistance to other Verticillium species which are prevalent among most tomato varieties. It is important to remember that there is currently no source of resistance to this disease in commercial varieties and the process of introducing resistance will take time , therefore the key method for dealing with this disease is chemical treatment of the soil. From the experience gained so far we can say that sterilization with methyl bromide is very effective in eliminating the pathogen of this disease.  Without this option, it is possible to treat the soil with metham sodium substances combined with solar sterilization (for a minimum of one month) to achieve effective sterilization. The symptoms of the disease are drying of the bottom leaves of the plant and if the stem is cut, dark, discoloration may be seen in the plant's veins.  Growers who suspect that the disease has appeared in one of their plots are required to inform us so that we can conduct laboratory tests to verify the disease (Fig. 1 – Symptoms of Verticillium Wilt of tomato). 

Clavibacter michiganense is a serious threat to tomato crops in Israel and worldwide. Should a plant be suspected of being contaminated with the disease, we recommend that the grower inform us so that we can verify such suspicion.  The disease pathogen is a bacteria that spreads through the plant's vascular system and harms all parts of the plant. Lesions first appear on the leaves of contaminated plants (see Fig. 2) and later, brownish streaks appear along the stems. These streaks crack open and rot sets into the vascular system. The top of the plant wilts. The leaves develop typical symptoms: Half the leaf wilts while the other half remains vital for a longer time. When the leaf is broken off, typical horseshoe shape discoloration is found at its base (see Fig. 3).   Finally, the entire plant wilts.  Under dry conditions, the bacteria's survival rate is high.  The bacteria is liable to remain in the soil and on solanaceous cultivars, such as peppers and eggplant, and weeds.

Plants can become infected by way of two primary methods:

A – Planting in contaminated plots. The disease pathogen accumulates in the soil and when the level is sufficiently high, the plants become infected.  In crops grown in soilless substrate, the water flow may carry and spread the disease.

 B – The disease is primarily spread by transmission of the disease from the foliage of contaminated plots through mechanical damage to plants caused by agro-technical operations of workers such as pinching of (shotse) plants, twisting of stems and removal of bottom leaves, whereby in most cases no symptoms of the disease are visible on the plants. 

How can plant contamination with the disease pathogen be prevented?  

A. Monitor suspect plants (yellowing or wilting plants). Clearly mark the suspect plants and do not touch them, unless at the end of the work day (in order not to infect healthy plants). In plots with isolated contaminated plants, they should be removed from the plot and destroyed (uproot the plant and place it into a plastic bag in order not to spread infected parts throughout the plot).

B. A contaminated area or plot should be treated separately. A separate team of workers should be formed for the contaminated plots. Work in suspect plots should be carried out only at the end of the work day, and in any event, work on plants in structures should be carried out using disposable gloves and special cutters or knives designated for use in said structure, and which are not used in other structures.

C. Prevent the entry of visitors into healthy or contaminated plots. Equipment for sterilizing shoes should be placed at the entrance to each structure – a tray with a flat sponge soaked in 2% solution of Sodium Hypochlorite , commonly known as bleach or similar solutions).

D. Sterilization of tools with 2% bleach solution.

E. Avoid using used packaging materials, do not move picking containers from contaminated plots to healthy plots. When using plastic picking containers, they should be rinsed or treated with 2% active chloride solution.

 

Western Flower Thrips (Frankliniella occdentalis)

 

This pest is becoming dominant in tomatoes and has recently been observed in several tomato plots covered with 50 mesh insect nets in various regions in Israel and worldwide.  TSWV damage which is transmitted by this pest has also been observed (see Fig. 4.). Most of the damage can be seen in the spring when the population of thrips in nature is high, however since the population becomes established inside a greenhouse, the activity of the thrips is liable to continue in the summer as well. Zeraim Gedera has several varieties with TSWV resistant which maintain generally accepted agro-technical qualities such as Tovi Cala and Baron. in any case it is important to control Thrips population by pesticides application such as abamectin abamectins Spinosad, emamectin benzoate. 
 

Large populations of this pest are found in israel from July until the end of October.  It is considered an extremely problematic pest due to the blackening damage to fruit and because it is a vector of various viruses such as TYLCV and other, geminivirus group. Today, Zeraim Gedera has several greenhouse varieties with TYLCV resistant which maintain generally accepted agro-technical qualities, the most prominent being Tovi Star, Allegro, Linda, and Tovi Cala.  in any case, it is inportant to control whitefly population by pesticides application especially during the first month of growth, of 1-3 treatments per week, followed by once every week/two weeks (according to the infestation and age of the plant) and to switch between the different groups of pesticides in order to prevent loss the resistance of the varieties and generation of large populations of whitefly which are resistant to pesticides.  Group 1: Nicotine derivatives (imidacloprid, acetamiprid, thiamethoxam, thiacloprid,dinotefuran) Group 2: diafenthiuron. Group 3:  thiocycllam hydrogen oxalate (caution: do not use more than twice as it may burn). Group 4: azadirachtin, , pyrethrins +neem oil (effective only for caterpillars)

 

The mites can first be found in the areas closest to the nets. Early detection prevents unnecessary spraying. It is recommended to spray against adult mites in response to initial appearance with one of the following groups of pesticides, which are effective against both types of mites, in rotation: abamectin, spiromesifen, pyrimidifen. The pesticides: tebufenpyrad, amitraz, azocyclotin, ,bifenazate,  are effective against spider mites. The pesticides: etoxazole, clofentezine, effectively control only the eggs of spider mite, and it is recommended to combine it with pesticides that are effective against adult mites.  

Leaf Miner (Liriomyza trifolii ).

This pest is very active in israel during the spring and can cause much damage (see section on early blight). Should this pest appear, it can be treated with one of the following pesticides: Abamectin, thiocycllam hydrogen oxalate (caution: do not use more than twice as it may burn), cyromazine, Spinosad (also effective against small caterpillars and thrips).

Important notice: Before every application carefully check that the pesticide is registered for the crop. If the crop is designated for export, it is imperative to check with the marketing company whether the pesticide is permitted, the duration of the waiting period prior to harvest and whether it is permitted to mix the pesticide with other substances or with natural predators in the greenhouse.  The advice given in this pamphlet has been prepared with the utmost care. However local circumstances and conditions greatly affect the final results of a crop. Therefore, Zeraim Gedera cannot accept any responsibility for the outcome of a crop.