SA Graan Julie 2016

Julie 2016

Research evolution

on soilborne diseases of maize

ften when interacting with producers, it becomes very

apparent that root and crown rot are rarely seen as a prob-

lem experienced by producers. The most common state-

ment being in the general lines of having successfully

practiced maize monoculture for 15 years and never struggled with

root rot a single day…

They would all agree that there were one or two seasons when

the yield achieved wasn’t satisfactory, but are quick to point a finger

to the lack of (or untimely) rain experienced during the seasons in

question. Upon closer enquiry, however, it becomes very clear that

no effort was made to dig up some plants and observe the degree

of root rot experienced during the season.

The question now arises as to how certain are you that the lower

yields can be attributed to climate and not root rot? A study con-

ducted by the ARC-Grain Crops Institute (ARC-GCI) demonstrated a

1,8 t/ha yield decline for every 25% increase in maize root rot.

Even with limited root rot, you accordingly already suffered some

losses. While not much can be done with regard to the climate, root

rot on the other hand can be managed.

Root and crown rot are seen as a disease complex as numerous

fungi are involved, whilst every plant will have some degree of root

and/or crown rot during the season. This fungal complex observed

in a specific field will in addition not only differ from the next, but

will also differ between seasons.

Graph 1

provides a profile of fungi isolated from maize roots sam-

pled at an ARC-GCI trial at Buffelsvallei (Ventersdorp). The various

bars depict the frequencies of twelve selected fungi as measured

over a period of five years. For this specific field,

Fusarium oxyspo-

rum, F. verticillioides

and

Trichoderma

spp. were the most promi-

nent over all seasons.

During the 2012/2013 season (indicated in yellow)

Stenocarpella

maydis

(causal organism of Diplodia stalk rot) as well as

Macropho-

mina phaseolina

(causal organism of charcoal rot) occurred in

higher frequencies than was observed in any of the other seasons.

The degree of root rot observed and the resultant yield loss expe-

rienced during this season correlated with these two fungi and the

conclusion was accordingly made that losses suffered could be at-

tributed to the impact their infection of the roots, crowns and stalks

had on the plants’ capability to fill their cobs.

In the past, such research was very labour intensive and time

consuming and to provide a similar service to producers was some-

what challenging. Recently, however, qPCR protocols were devel-

oped for twelve of the most common occurring fungi on maize roots,

crowns and stem by the ARC-GCI.

These fungal isolates include

Pythium

spp. (seedling blight/death),

Rhizoctonia solani

(damping off/failure to geminate/seedling blight),

Curvularia eragostidis

Macrophomina phaseolina

(charcoal rot),

Exserohilum pedicellatum

Phoma

spp. (root and crown rot/red root

rot),

Trichoderma

spp.,

Fusarium chlamydosporum

F. oxysporum

F. equiseti

and

F. verticillioides

(organisms causing Fusarium root,

crown and stalk rots) and

F. graminearum

(Gibberella root, crown

and stalk).

This technology enables researchers to accurately identify and

quantify these fungal pathogens without having to plate out and

perform microscopic identification and sequencing. During the past

season this method was tested on some of the root samples sent

in by producers.

In order to obtain the best possible explanation for visual defects

observed in the field, samples brought for analysis must both

consist of plants that are seen as ‘healthy’ (or control plants) as op-

posed to those that are seen as ‘diseased’.

Photo 1

presents such a sample where the ‘healthy’ plant had

clearly a much larger root system than the ‘diseased’ plant. In the

field, the ‘diseased’ plant appeared stunted. What should, however,

be noted is that both root systems had root rot to varying degrees.

The results obtained from qPCR analysis showed that mainly three

fungal pathogens were detected:

Fusarium equiseti, Exerohilium

pedicellatum

as well as

Rhizoctonia solani

(

Table 1

and

Photo 2

Again it should be noted that both root systems had varying degrees

of certain fungi, but the ‘diseased’ plant had much higher frequen-

cies of

F. equiseti

compared to the control plant. Now, at least, we

have an idea of which pathogen(s) are present within the production

system, which might be responsible for yield losses suffered.

Based on the fungi identified, recommendations can be made for

the way forward. With the current case, the results were somewhat

confusing as

E. equiseti

is known to result in root rot in younger

plants, whilst other international literature view the fungus more as a

saprophyte (i.e. an organism that survives on dead organic material).

The recommendation was accordingly made that crop rotation

should be considered (since the plants came from a maize mono-

culture system), but also that the plants and soils be tested for the

presence of nematodes. What is often observed is that the damage

caused by nematodes will result in greater degrees of infection by

fungi in the surrounding soil.

The observed root rot and the subsequent fungi isolated is accord-

ingly rather secondary than the primary cause for the visual damage

observed. It is therefore possible that other factors can predispose

plants to infection by fungal pathogens. Nematodes can play an

important role in soilborne diseases of crops and should not be

overlooked in disease enquiries. Another such factor is herbicide

damage. Soybean samples were sent in with symptoms that resem-

bled that of Pythium root rot (

Photo 3

International literature, however, warns that symptoms of Pythium

root rot on soybean and herbicide damage are nearly identical and

care must be taken with the identification of the disease. Plating

out the root samples allowed the growth of many fungi, but no

Pythium

spp. were identified. No amplification of

Pythium

spp.

could in addition be obtained through qPCR analysis and Pythium

had to be eliminated as the possible culprit.

Due to the wide range of other fungi isolated, primary pathogens

could not be identified. This created the impression that the rot

observed was a result of random infection by whatever fungi were

present in the soil. The conclusion was drawn that although root rot

was present and could have resulted in the losses observed, the

ON FARM LEVEL

Maize / Root rot / Crown rot

Integrated pest control

ANEEN SCHOEMAN

and

MARYKE CRAVEN,

ARC-Grain Crops Institute, Potchefstroom