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Friday, 08 July 2022 09:58

Brassica Stunting Disease

Written by DR B.A. CAMPBELL, SOIL SCIENTIST - CHARTER SEEDS
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Brassica Stunting Disease (BSD) was identified in Bulawayo some 4 years ago by Talha Loonat, Starke Ayres Brassica Specialist. 

It has since become a major threat to brassica production in Matabeleland. I have not yet seen it in Mashonaland. 

It is also a major problem in RSA. The first outbreak was the Brits area and within a year spread all over the country with the exception of the Western Province. 

So serious did the disease become that a major study was carried out by four major seed companies of which Starke Ayres was one. 

It took a considerable period of time to finish the study because the symptoms are completely different to any other cabbage disease and it cannot be transferred mechanically or cultured. It cannot be transferred via the seed nor is it present in soil or water. 

 

Two Polero viruses were identified in the diseased plants. The viruses are related to Turnip Yellowing Virus (TuYV) which is widespread in Europe but more severe in Southern Africa. 

The virus is spread by aphids. A number of species are involved but the main vector appears to be the Peach aphid (Mysus persicae). 

Once the virus is identified in a district, the problem becomes one of aphid control because there is no other way for the virus to spread.

Aphids can be either winged or wingless. The winged form is transient and is responsible for the primary infection. The wingless form is responsible for the secondary infection and spreads the virus to adjacent plants in the crop. Some species like the cabbage aphid (Brevicoryne brassicae) can produce up to 41 generations of females in a season. 

 

In any given season there is a strong correlation between the population of winged aphids and the incidence of BSD

Infection seems to hit a bi-annual peak (every second year) in line with aphid populations. 

Once an aphid becomes infected it remains so for the rest of its life. 

 

There are no resistant or tolerant varieties of cabbages, but broccoli and cauliflower are less susceptible than cabbage. 

Symptoms of the disease are severe stunting, flattening of the leaves, purpling of the leaves, blackening of the vascular tissue, reduced head size, or no head formation, depending on time of infection. 

The black ring around the stem is caused by deposition of tannin and phenolic products in the phloem. 

The first visible symptom becomes apparent 4-6 weeks after transplant. 

Aphids and polyphagous - they are not confined to one kind of crop. 

 

Currently Bulawayo is a network of small plots, growing a wide variety of green crops. It should be noted that even if crops are not brassicas (e.g. green mealies) the aphids generated on that crop can become infected subsequently, or even on the crop itself, and make a mild infection much worse. 

 

Aphids tend to move when a host plant dries off. 

Scouting is of limited importance. Severe infections have been seen in fields where no aphids were seen. 

If a brassica crop is contemplated, the farmer concerned should first establish what other crops will be grown in the area. 

 

Cruciferous weeds must also be controlled. 

Currently in Zimbabwe little attention is paid either to rotations or isolation distances. 

 

There is no recommended isolation distance. Aphids can be blown a long way by the wind.

 

It is very important that seedlings are treated with a systemic insecticide in the nursery.

 

A comprehensive spraying program must be instituted from the nursery onwards.

In the nursery spray Actara (thiamethoxam) followed by a drench of Confidor (imidacloprid) and followed in turn by regular weekly or twice weekly applications of registered proven aphicides.

 

In RSA yellow bucket traps are used to give an indication of aphid pressure on a field. #farming

#Charterseeds

#SeedsOfSuccess

 

Read 293 times Last modified on Friday, 08 July 2022 10:12

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