Phytoplasma and virus problems in soybean and mungbean crops

tiny, sterile soybean pods

Pho­tos 1 and 2: Tiny, imma­ture, ster­ile pods on soy­bean from Dar­ling Downs.

A dev­as­tat­ing dis­ease was observed in sev­er­al pad­docks of soy­bean crops in the Branchview area of the Dar­ling Downs in late autumn 2016. Almost 100 per­cent of plants were affect­ed in some pad­docks. The affect­ed plants pro­duced no, or very few filled pods and instead had a pro­lif­er­a­tion of tiny imma­ture pods as shown in Pho­tos 1 and 2. The plants also remained green while near­by, unaf­fect­ed crops matured and browned off as nor­mal­ly expect­ed.

The severe­ly affect­ed crops had so few filled pods that they were uneco­nom­i­cal to har­vest. Dar­ling Downs agron­o­mist, Matthew Hold­ing, report­ed that yield loss­es were very high across two farms. With symp­toms only becom­ing severe late in the crop cycle, the major­i­ty of the crop input costs, such as irri­ga­tion and weed con­trol had already been com­mit­ted to the crops before the dam­age was appar­ent, result­ing in sig­nif­i­cant eco­nom­ic loss­es. Mr Hold­ing alert­ed Queens­land Depart­ment of Agri­cul­ture and Fish­eries, and Uni­ver­si­ty of South­ern Queens­land plant pathol­o­gists and ento­mol­o­gists to the issue in May but no link was found with insect dam­age, viral or fun­gal dis­eases. How­ev­er, about 70 per­cent of the affect­ed plants test­ed pos­i­tive for the pres­ence of a phy­to­plas­ma using lab based mol­e­c­u­lar diag­nos­tics. Phy­to­plas­mas are spe­cialised bac­te­ria that sur­vive with­in plants and the sap-suck­ing insects that vec­tor them, leafhop­pers and plan­thop­pers. These sap-suck­ing insects are the only insects that can trans­mit phy­to­plas­mas and they may pick up the pathogen from near­by weed hosts before mov­ing it into crops. There is a wide range of phy­to­plas­mas and they cause a vari­ety of symp­toms in dif­fer­ent hosts but legume crops often dis­play lit­tle leaf symp­toms and steril­i­ty of flow­ers.

The observed pod symp­toms from the Dar­ling Downs were sim­i­lar to typ­i­cal phy­to­plas­ma symp­toms but the lack of lit­tle leaf symp­toms was unex­pect­ed and the extent of dam­age across entire pad­docks has not been observed before. While we have not yet been able to demon­strate the detect­ed pathogen can cause the observed symp­toms under con­trolled con­di­tions, we believe that the phy­to­plas­ma was the cause of the dis­ease out­breaks on the Dar­ling Downs. The lab test results indi­cate the phy­to­plas­ma is close­ly relat­ed to pigeon pea lit­tle leaf phy­to­plas­ma, which has been pre­vi­ous­ly detect­ed in Aus­tralia in pigeon pea and sty­losan­thes.

Brown leaf hopper

Pho­to 3: Poten­tial phy­to­plas­ma vec­tor – the brown leaf hop­per (Oro­sius ori­en­tal­is).

The extent of dam­age caused by this out­break was not typ­i­cal and we urge grow­ers and agron­o­mist to be on the look­out for these symp­toms in the com­ing sea­son. It is not cer­tain that the dis­ease will reap­pear in the com­ing sea­son, but if it does, we will need to con­tin­ue inves­ti­ga­tions includ­ing which insect species is respon­si­ble for trans­mis­sion, its host range and pos­si­ble con­trol options. One poten­tial vec­tor is the brown leaf hop­per Oro­sius ori­en­tal­is (Pho­to 3).

 

While soy­bean aphids were present in the affect­ed Downs crops, it is extreme­ly unlike­ly they were respon­si­ble because aphids are not phy­to­plas­ma vec­tors, and soy­bean aphid attack has nev­er been observed to lead to the small bunchy pod symp­toms seen on the Downs. How­ev­er, heavy soy­bean aphid infes­ta­tions can also delay crop matu­ri­ty (Pho­to 4).

aphid damage symptoms

Pho­to 5: Typ­i­cal aphid dam­age symp­toms in flow­er­ing soy­beans show­ing includ­ing dis­tort­ed new leaves sticky with hon­ey dew. See also white skins shed by moult­ing aphid nymphs.

Crop greening

Pho­to 4: Crop green­ing due to soy­bean aphids. Unsprayed soy­bean plants clos­est to view­er, show delayed matu­ri­ty (green plants), con­trast­ing with the har­vest-ready main crop which was sprayed for soy­bean aphids.

 

soybean aphid

Pho­to 6: Soy­bean aphid adult and nymphs.

Grow­ers and con­sul­tants should mon­i­tor soy­beans for this pest from the ear­ly veg­e­ta­tive stages onwards. Action should be tak­en if pop­u­la­tions exceed thresh­old by flow­er­ing, the thresh­old being 250 aphids per plant.  As a rule of thumb, aphids are above thresh­old if aphids can be seen on the plant’s stem.  Also look for dis­tort­ed new growth leaves with sticky hon­ey dew (Pho­to 5), small bright green aphids and their cast-off skins shed by moult­ing nymphs (Pho­to 6), as well as preda­to­ry lady­birds, which are good aphid indi­ca­tors (Pho­to 7).

ladybird

Fig­ure 7. Aphid infest­ed soy­bean crop with lady­birds read­i­ly vis­i­ble but aphids not so obvi­ous.

If spray­ing is war­rant­ed, the pre­ferred option is pir­im­i­carb for which a per­mit renew­al is pend­ing (pre­vi­ous­ly PER 13451.) Pir­im­i­carb is the pre­ferred option because it has no impact on key aphid preda­tors includ­ing lady­birds and hov­er­fly lar­vae.

At about the same time as the phy­to­plas­ma dis­ease out­breaks on the Dar­ling Downs, sev­er­al mung­bean crops in the Spring­sure and Gin­di regions of cen­tral Queens­land had high num­bers of plants dis­play­ing typ­i­cal phy­to­plas­ma symp­toms with lit­tle leaves (or some­times called witch­es broom) and steril­i­ty of flow­ers (Pho­to 8). This result­ed in sig­nif­i­cant loss­es in crops with high inci­dences of affect­ed plants. It is not clear if it was the same phy­to­plas­ma caus­ing the dis­ease out­breaks in cen­tral Queens­land and on the Dar­ling Downs, but per­haps there were wide­spread con­di­tions favourable for leafhop­pers to pick up the pathogen from weedy hosts and move through the crops in both regions.

phytoplasma

Pho­to 8: Typ­i­cal phy­to­plas­ma symp­toms on mung­bean from cen­tral Queens­land crops.

A recent detec­tion of Cow­pea mild mot­tle virus in soy­bean and French bean in South East Queens­land has also caused sig­nif­i­cant con­cern for bean grow­ers. It caused wide­spread loss­es for French bean grow­ers in the Fas­sifern Val­ley and alarm­ing­ly has also been report­ed to cause sig­nif­i­cant loss­es in soy­bean crops over­seas. This is a new virus to Aus­tralia, trans­mit­ted by sil­ver leaf white­fly and in the seed of some hosts. It is not clear how long it has been in Aus­tralia or if it will cause dis­ease out­breaks in soy­bean pro­duc­tion areas but grow­ers and agron­o­mists are urged to keep a look out for this virus. Symp­toms in soy­bean include dis­tort­ed leaves with obvi­ous mot­tle (Pho­to 9).

CMMV

Pho­to 9. Dis­tort­ed leaves and mot­tle symp­toms of cow­pea mild mot­tle virus on soy­bean from Lock­y­er Val­ley crop.

If you have seen symp­toms of Cow­pea mild mot­tle virus or phy­to­plas­ma and you live in Queens­land report it to the Biose­cu­ri­ty Queens­land on 13 25 23. Images and text may be sent to [email protected]. For all oth­er areas con­tact the nation­al Exot­ic Plant Pest hot­line on 1800 084 881.

Arti­cle con­trib­u­tors: Mur­ray Shar­man, Hugh Brier, Fiona Filar­do, Sue Thomp­son, Matthew Hold­ing, Denis Per­s­ley